Idiomaticity

Idiomaticity is variously defined and described. The following attempts to single out the two main features of what constitutes and idiom in a language and then to investigate how variable the features are.
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What springs to mind when you see these images? Click here when you have thought of 6 expressions.

You should have thought of:

bite the bullet (suffer an unpleasant and unavoidable situation)
I'm all ears (I'm listening carefully and am interested)
a piece of cake (an easy and enjoyable task)
burn the candle at both ends (work late into the night and begin again in the early morning, thus exhausting oneself)
let the cat out of the bag (reveal a secret)
pull someone's leg (play a joke on someone by telling a lie)

There are two important characteristics of the expressions in black. What are they?
Click here when you have an answer.

non-compositionality: This simply means that you can't get the meaning of the expression even if you know all the words in it. There is no way (without clear context) to guess, e.g., what let the cat out of the bag means.
Another way of saying this is that idioms are often opaque in meaning on the scale of fully transparent to completely indecipherable. More follows on the relative opaqueness of idiomatic language.
This is not to say, of course, that the expressions have no literal meaning. It is possible, quite literally, to let a cat out of a bag but an English speaker would know immediately from the co-text and context whether an expression is being used idiomatically or literally at any time.
fixedness: All the examples of idioms above are fixed: you can't replace any of the words (except someone in the last case with a name or pronoun), you can't leave out any of the words and you can't insert words.
You can, naturally, bite a stick, burn a hurricane lamp, let the donkey out of the stable, release the cat from the bag, bite the 6mm bullet and pull someone's ear but none of the expressions will have the same meanings as these examples.
Fixedness is a relative term, as is opacity. Read on for a discussion of what you can and cannot alter in idioms.

For a little more on semantics and noncompositionality, consult the guide to semantics linked in the list of related guides at the end.

So, idioms are non-compositional, fixed expressions. End of story, right?
Not quite, no.

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Definitions

An early definition of an idiom comes from the linguist and teacher Henry Sweet (a major influence in the Reform Movement's reaction to grammar-translation approaches, incidentally). He stated:

The meaning of each idiom is an isolated fact which cannot be inferred from the meaning of the words of which the idiom is made up.
(Sweet, 1889:139)

Since Sweet's time, the area has been continually revisited by researchers interested in finding out what constitutes idiomaticity in languages and how the various types of idiom can be classified and analysed. What we have ended up with is a confusing muddle of terms, definitions and classifications which is, to say the least, unhelpful. You may, for example, come across any or all of the following terms if you research this area:

figurative idioms or non-compositional metaphors	to refer to fact that we can often find a connection between figurative, idiomatic and literal meaning. It is for example, just possible to figure out what bite the bullet might mean with some knowledge of pre-anaesthetic surgery. Ditto, perhaps, with have an ace up one's sleeve
binomials	to refer to expressions such as time and again, Ladies and Gentlemen which occur as pairs of words, often with a fixed ordering
fixed expressions	to refer to idioms which are truly fixed such as an open and shut case
semi-fixed expressions	to refer to idioms where some flexibility is allowed. For example, you can throw in the towel but also throw in the sponge, both meaning surrender, and both derived from boxing
lexicalised expressions	to indicate that the expression functions as a single lexeme. For example, kick the bucket actually just functions as the verb die
opaque expressions	emphasising the fact that is often not possible to work out metaphoric meaning from literal meaning as it is with figurative metaphors. For example, chew the fat
frozen collocations	emphasising the fixedness characteristic of some idioms such as a can of worms
restricted collocations	referring to those which allow some flexibility but only within a limited range. For example, you can be a big/large/huge fish in a small/little/tiny pond
semi-idioms	to refer to anything which seems like an idiom, insofar as it acts like a single word, but is not completely opaque and fixed. One part of the expression has a figurative meaning not found elsewhere but the other part is 'normal' as in expressions such as pay attention or foot the bill

It is not the suggestion here that such refinements are useless or deliberately confusing but we are interested in classifications which will be useful to us as English-language teachers rather than research linguists so this guide will focus on two the central characteristics of idiomatic language: fixedness and opacity (or non-compositionality, if you prefer).
This will be at the expense of some precision so if you are looking for more, there are references at the end to on-line, more scholarly articles that you may want to read.
In some analyses, the definition of an idiom includes both fixedness (the inability to change any of the components) and opacity (non-compositionality) but both these phenomena exist on a scale from fully fixed and opaque through semi-fixed and opaque to variable and easily understood. The definition soon breaks down.

Idiom, collocation, lexical chunks and, phrasal verbs: a blurred area

There are three areas in which the distinction between idioms in the true sense and other word combinations is not distinct.
In some analyses, the definition of idiom is so broad as to encompass most of the language. For the sake of sanity, the following are not considered here as representing true idioms.
They have their own guides on this site which you will find linked in the list of related guides at the end.

Collocations

In the list above, we have something called semi-idioms and the examples are pay attention and foot the bill in which only one part of the expression is used figuratively.
There is a range of verbs which act in this way, collocating with nouns in predictable and fixed patterns. For example:
    find fault
    lose confidence
    make arrangements
    pay a compliment
    take notice
    win respect
    make an excuse
    do justice
    lend a hand
    gain admiration
and, although the verbs will collocate with a range of other nouns, in these meanings there is usually only one possible choice. We cannot, for example have:
    *say excuses
    *do arrangements
    *borrow a hand
    *discover fault
etc.
Some of the combinations do allow a certain flexibility so it is possible to have:
    win admiration
    gain, achieve or earn respect
These items are semi-idioms insofar as only one feature of the definition of an idiom applies. The verb in such expressions is often described as delexicalised although in some the verb's usual meaning plays a minor role.
Because the nouns are usually transparent in meaning, they are not particularly opaque and the meaning can be derived from understanding their components. In that sense they are not true idioms.
It is also the case that some verbs may be considered only semi-delexicalised in that their meaning does contribute something to the overall meaning of the clause in which they appear. For example, in:
    pay a compliment
    take an interest
    set an example
    catch a name
    throw a tantrum
there is some sense of the verb's usual meaning in the clause but it is still very hard to predict which verb will form an acceptable collocation so the expressions may be considered idiomatic in that respect.
They are, however, more or less fixed as we saw and in that respect they are idioms.

It is also the case that delexicalised verbs can function almost wholly idiomatically. For example:
    Take the bucket to the tap
contains a use of take which is wholly transparent and non-idiomatic. However:
    Take a hammer to it
is an idiomatic use of take which is non-transparent because it means something like:
    Hit it with a hammer
And in:
    They made him the manager
we have a non-idiomatic use of make but
    If he's lived here since the war, that makes him over 90
is an idiomatic use of make which is akin to:
    If he's lived here since the war he must be over 90

For a list of semi- and fully delexicalised verbs, click here.
There is also a lesson for B1 / B2-level learners on delexicalised verbs here (new tab).

We also encountered what some people call frozen collocations which are so strong that they act as single words, always appearing in combination. The example was a can of worms but many partitives are restricted in their collocations and we also find, e.g.:
    a rasher of bacon
    a pane of glass
    a gust of wind
and so on which can only occur in the context of the commodity or substance they describe. For more, see the guide to classifiers and partitives, linked below.
Other very strong collocations which are produced and understood as single concepts and not constructed from their component parts include, for example:
    vested interests
    rock solid
    special pleading
    heart rending
and so on.
Whether such expressions are considered idioms (because they are clearly idiomatic) or just very strong collocations is a matter of taste and authorities differ.
When the collocation is noun + noun, some will shade into compound nouns so, for example:
    light bulb
    tree surgeon
    garden rake
    baby sitter
are all analysable as compound nouns and may even be written as one word.
In that sense they are not idioms because it is a simple matter, usually, to add meaning one to meaning two and arrive at the meaning of the whole expression.

The upshot of all this is that there is probably no way in principle that one can draw a line between a reasonably strong collocation and an idiom although it an easier task to identify one or the other.

Lexical chunks

We owe the term lexical chunk to Lewis (1993) and his investigation of a lexical approach to teaching. He suggests that there is a range of more or less fixed chunks in the language including:

polywords, e.g.:
    inside out
    back to front
    in that case
    if only
institutionalised utterances, e.g.:
    Sorry to have kept you
    Take my word for it
    Would you like ...
    I'll see
    And that's that
sentence frames, e.g.:
    That's not as ... as you think
    In what follows ...
    The upshot was ...
    And quite predictably ...

Lewis also focuses on the fact that some verbs, as we saw above, take their meaning from the noun with which they form a strong collocation. This is what he calls delexicalisation and it mostly affects:
do | have | get | go | make | put | set | take
although we can add other marginal cases such as earn, pay, run and save.
All such multiple-word units (or MWUs in the trade) certainly exhibit some fixedness and are probably produced and understood as single items but they are not opaque in meaning. They are idioms in the first sense but not in the second.
You may, incidentally, also find such chunks described as holophrases, prefabricated routines or formulas.

Phrasal verbs

One definition of a phrasal verb is that the adverb particle combines with the verb to produce a meaning which cannot be retrieved from understanding the sense of the adverb and the sense of the verb: a third meaning needs to be discovered.
Clearly, for example:
sit back
    sit down
    sit up
    sit in
    sit out
are all verbs post-modified by an adverb but the meaning is extractable by understanding both parts so they are not phrasal verbs or idioms. Changing the adverb has no effect on the meaning of the verb at all.
However,
    sit by (fail to take action)
    sit on (suppress)
    sit around (be idle)
    sit up (take notice)
are not easily understood by understanding their elements so count as idiomatic because they are both fixed (in the senses used) and opaque. In this case, changing the particle will affect the meaning of the verb because it is the combination which supplies the third meaning.
Again, of course, there are degrees of opacity, but not of fixedness, because it is possible to understand the meaning of many by a small leap of imagination from the usual meaning of the prepositional use of the particles. For example, once one has grasped that one of the meanings of out is clear or loud then combinations such as:
    speak out
    shout out
    make out
and more are quite easily understood as are:
    get on
    go on
    move on
    walk on
once we understand that one of the meanings of on is make progress.
Opacity is consequently low.
Phrasal verbs have their own section on this site (as part of the consideration of multi-word verbs in general) so will not feature in this guide to idiomaticity. See the link at the end for more.

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Origins

Although this distinction is not necessary for teaching purposes, it is sometimes helpful in terms of memorisation to know the origin of the idiom one encounters. There are two possible sources (which often overlap):

Fixed metaphor

Frequently, very influential texts in English, such as the bible, Shakespeare's works and others, contain metaphors which have come into everyday use and become fixed idioms. Some metaphorical uses are obscure in origin. Examples include:
    heart of gold
    laughing stock
    wild goose chase
    wear your heart on your sleeve
    a foregone conclusion
    be cruel to be kind
    salad days
    cold comfort
    a tower of strength
(all from Shakespeare)
    at the eleventh hour
    by the skin of your teeth
    a millstone around your neck
    the writing on the wall
(all biblical)
    it gives me the creeps
    go on the rampage
(both from Charles Dickens)
    spill the beans
    straight from the horse's mouth
    let the cat out of the bag
    count your chickens before they are hatched
    out to lunch
    just my cup of tea
(all obscure in origin but some may conveniently but speculatively be derived from some occupations)
Historical and specific register origins

Other idioms derive from certain registers: sport, the military, trade, sailing and so on and are often opaque without a certain amount of knowledge of the history of the domains. They are usually not completely opaque, however.
Examples include:
    a sticky wicket (from cricket)
    cover all the bases (baseball)
    run with the ball (rugby or American football)
    game, set and match (tennis)
    a level playing field (many sports)
    plant a seed (horticulture)
    plough through (farming)
    cut a deal (obscure but possibly from card games)
    hit a snag (angling or river navigation)
    a loose cannon (from naval warfare)
    flash in the pan (musketry)
    close ranks (army parade term)
    half-baked (cookery)
   cut and dried (herbalism)
    sail close to the wind (sailing)
    learn the ropes (sailing)

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	Proverb origins
Let sleeping dogs lie

Many idioms derive from proverbs in English although they are often truncated because the proverb is well known and does not need to be said in full for the message to be clear.
Such expressions are almost always very opaque because one needs access to the whole idiom to understand the meaning. Examples include:

Abbreviated idiom	Source proverb	Meaning
Don't blame your tools	A bad workman always blames his tools	Poor workers will never blame themselves
Well, we have a bird in the hand	A bird in the hand is worth two in the bush	Be satisfied with what you have secured
He's the weakest link	A chain is only as strong as its weakest link	One weak element will make the whole thing weak
You are clutching at straws	A drowning man will clutch at a straw	When in desperation we will look anywhere for help
She won't change her spots	A leopard cannot change its spots	People are reluctant or unable to change their behaviours
All's well that ends well	Usually quoted in full	If the result is good, past problems don't matter
All that glitters	All that glitters is not gold	Things may appear better and more valuable than they really are
He's an empty vessel	An empty vessel makes the most noise	Ignorant people are often the most talkative
A stitch in time	A stitch in time saves nine	Repair something before it gets worse
He's a bit of a rolling stone	A rolling stone gathers no moss	People who move between places and jobs don't get rich
It's in the eye of the beholder	Beauty is in the eye of the beholder	Attractiveness is a personal opinion not reality
Beggars can't be choosers	Usually quoted in full	If you get something free, you can't complain about the quality
Better late than never	Usually quoted in full	As long as it's what you want, lateness doesn't matter
Curiosity killed the cat	Usually quoted in full	Do not pry
It's all coming home to roost	Curses, like chickens, come home to roost	The consequences of your actions will come eventually
Pearls before swine	Do not cast pearls before swine	Do not give people better than they can appreciate
We'll cross that bridge later	Do not cross the bridge till you come to it	Do not waste time worrying about a future that may not happen
Don't kill the goose	Don't kill the goose that lays the golden eggs	Don't destroy something that is successful
That's the silver lining	Every cloud has a silver lining	Even bad situations have some good in them
That's just spilt milk	Do not cry over spilt milk	Don't worry about misfortune which cannot be changed
Make hay	Make hay while the sun shines	Take advantage of good conditions while they are here
Once bitten	Once bitten, twice shy	People will not repeat actions which were unpleasant
The grass is always greener	The grass is greener on the other side of the fence	People always want more than they have
We don't want too many cooks	Too many cooks spoil the broth	Having too many people in a team confuses a task
When in Rome	When in Rome, do as the Romans do	Adjust your behaviour to suit the culture you are in
You can lead a horse to water	You can lead a horse to water but you can't make it drink	You can show people how to do something but you cannot force them to do it
That's the last straw	It's the last straw that breaks the camel's back	A small increment can destroy all of something
Many hands	Many hands make light work	The more people you have the easier a job will be
You're an early bird	It's the early bird which catches the worm	The sooner you start the better will be the prospects

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Fixedness

This is not an on-off characteristic. Some idioms are more fixed than others, some are very flexible. Here's a cline for you to see what's meant. Where would you put the idioms on right in the cline on the left (if idioms they actually are)? Click on the image for some comments when you have an answer.

through thick and thin
hammer and sickle
aid and abet
have a blast
hit the sack
off one's rocker
call it a day
assets and liabilities
left, right and centre

life or death
back to the drawing board
cut corners
put all your eggs in one basket
torrential rain
wouldn't be caught dead
miss the boat
make the grade
make the beds
raining cats and dogs

On the left-hand end of the cline, we can arrange those idioms which are clearly fixed in which we cannot add words, replace words or delete words without changing the meaning entirely. These include:

hammer and sickle, aid and abet, life or death: These are fixed-order binomials, sometimes called frozen binomials. You can't reverse the order without sounding odd and neither part can be replaced to maintain the meaning. Nor can you insert anything between the parts. When they are nouns, such binomials are treated as singular in English (the hammer and sickle is, thunder and lightning was etc.).
They belong on the far left.
left, right and centre: This is similar but is actually called a trinomial (for a pretty obvious reason). Other examples are: blood, sweat and tears; cool, calm, and collected; win, place, or show; tall, dark and handsome and so on.
True trinomials like these belong on the left of the cline.
call it a day, cut corners, put all your eggs in one basket, wouldn't be caught dead, miss the boat, make the grade, raining cats and dogs, through thick and thin: These are all fixed idioms. The last of these (through thick and thin) is often classed as a fixed binomial. We can't vary the order of words, insert words, replace words or delete words in these expressions. We can't have
    put all your eggs in one tray
    miss the train
    rain cats and monkeys
    call it a busy day
    cut sharp corners
if we want to retain the meaning.
Note, however, that even with fixed idioms, we can alter the tense and must make the pronouns agree (they called / are calling it a day, he missed / will miss the boat, she'll never make the grade etc.) Other structural changes are possible but very rare such as cut a corner but most are forbidden such as *put all your eggs in baskets or *make the grades.
They belong close to the far left of the cline.

On the right we have the most flexible expressions and there are only two of these:

torrential rain, make the beds: These are strong collocations because there is a limited number of possible substitutions with verb + the beds or torrential + noun. We can, just possibly, have do the beds and make up the beds and have torrential waterfalls or torrential rivers but it's hard to find more replacements. The other way round, however, produces a wider range of possibilities. The verb make collocates with an enormous number of nouns (mistakes, haste, mess etc.) and the number of possible adjectives for rain is also large (heavy, light, drizzly, hard, thin etc.). However, some strong collocations are so predictable as to constitute real idioms. For example,
We pay attention, a compliment and our respects but take an interest, offence, place etc. and give explanations, thanks and promises etc.
It is quite arguable that collocations of this sort do not fully qualify at all as idioms and won't be considered further here. There is a separate guide to collocation on this site, linked below.

The rest of the expressions fall somewhere in the middle of the cline:

have a blast, hit the sack, off one's rocker, back to the drawing board: These all have alternates with the same or a very similar meaning so they are semi-fixed idioms. We can have:
    have a hoot
    have a whale of a time
    hit the hay
    off one's trolley
    back to square one
which all carry the same sorts of meaning, although they are sometimes stylistically different. Expressions such as these only have a very limited number of alternatives and they belong on the mid-left of the cline. They share other characteristics with true fixed expressions in not allowing deletions or insertions but a severely limited range of alternate words is possible.
assets and liabilities: This is rather an unusual case because it is a binomial but the ordering is not fixed (although this is the preferred one). Other examples of binomials which do not have fully fixed ordering include:
    salt and pepper
    see and hear
    friends and family
etc.
There is usually a preferred order with all binomials but the reasons for it are complex. See Benor and Levy (no date) for a technical discussion.

There is an interesting exception to fixedness.
Some idioms and even binomials which are normally considered wholly fixed can be modified with intensifiers. It is possible, therefore, to have:
    I have been rushing hither and bloody yon all day
    I smell an extremely large rat
    I've missed the damn/an important boat
    have a total/absolute/complete blast
etc. In particular, the so-called taboo words, bloody, bleeding, damn etc., are used in this way.

Flexibility

Idioms may be flexible to a certain extent, then, but the flexibility is also analysable by type.

Conjugation
Idioms which contain verbs are frequently conjugated to conform to the normal rules so, e.g.:
    pass the buck
may be
    passes, passed, is passing the buck
and so on and the verb can be nominalised as in:
    I resent your passing the buck
Pronouns will change in the normal way and may be accompanied by other changes, e.g.:
    She bit off more than she could chew
    He has bitten off more than anyone could chew
Passivisation
Such idioms can also be made passive so we can allow
They have cooked the books
and
The books have been cooked
Insertion of words (usually adjectives or adverbs)
    right at the eleventh hour
    teaching new tricks to a very old dog
    by the absolute skin of my teeth
Re-formulation
We need a level playing field → We need to level the playing field
She has a heart of gold → Her heart is of pure gold

Constraints

Some idioms are constrained structurally. For example:

Some can only be used in the negative so we allow:
    I didn't sleep a wink
    I can't make head (n)or tail of it
    She wouldn't be caught dead with him
but not:
    I slept a wink
    I made head or tail of it
    She would be caught dead with him
Some can only appear in the passive or active voice so we allow:
    He's fed up with the work
    I am snowed under with emails
    Don't beat about the bush
    Let's call it a day
but not:
    *The work fed him up
    *The emails snowed me under
    *The bush was beaten about
    *It was called a day
However, reformulation is often allowed so we see and hear:
    He missed the boat
    The boat was missed
    I gave him the benefit of the doubt
    He was given the benefit of the doubt
    The benefit of the doubt was given to him
    She was let off the hook
    They let her off the hook
and so on.

This is a wholly unpredictable area.

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Opacity or non-compositionality

Again, there's a cline because there are levels of opacity and transparency. The image below separates them into those whose meaning is obvious (literal), those where it can be deduced (figurative uses) and those which are wholly opaque (often metaphorical).
As you did above, locate these expressions on the right somewhere on the cline on the left and then click the image for a commentary.

barking up the wrong tree
a bitter pill to swallow
by the skin of one's teeth
bread and circuses
kiss and tell
heads or tails
spick and span
holding all the aces
at a snail’s pace

a dime a dozen
bite off more than one can chew
cut the mustard
bob and weave
under the weather
hot and bothered
research and development
come rain or shine
Tom, Dick and Harry

Very few of these idioms are going to be on the far left of the cline. If they were, they would hardly qualify as idioms at all.
So, we have the first group of idioms whose meaning can be worked out quite easily from their literal interpretation or whose meaning is literal, in fact.

heads or tails, a dime a dozen, research and development, at a snail's pace: These are the most literal. It is easy to work out what they mean from understanding their constituents. Many bi- and tri-nomials fall into this category:
    knife and fork
    law and order
    salt and pepper
    sick and tired
    red, white and blue
    sex, drugs, and rock 'n' roll
etc. Many do not have transparent meanings, however, such as
    I'm all fingers and thumbs
    It's neither fish nor fowl
Most idioms do not fall into this category.

The figurative section is the most crowded and it's a matter of opinion whether an expression falls to the left or right of the centre of the cline.

barking up the wrong tree, a bitter pill to swallow, kiss and tell, holding all the aces, bite off more than one can chew, hot and bothered, come rain or shine: In many of these sorts of expressions, only one of the items is used figuratively so it's possible to work out what's meant. In others, it is a short step from the literal to the figurative meaning. For example, imagining a picture of a dog barking up the wrong tree immediately allows you to make the jump to the figurative meaning of barking up the wrong tree.
Equally, if you know something about many card games, you'll know that holding all the aces is likely to lead to a win. Even kiss and tell needs little context for you to understand its meaning.
With binomials and some other idioms, it's possible to assume that the terms are synonyms or antonyms of sorts and understanding one allows you to understand the whole. hot and bothered and come rain or shine fall into this category.

Finally, there are the wholly opaque idioms:

by the skin of one's teeth, spick and span, cut the mustard, bob and weave, under the weather, Tom, Dick and Harry: All these are opaque and you'd have to be very lucky to guess their meaning even with context to help. Many idioms are like these and they have to be learned as single units of meaning.
There are also a number of common binomials in which one or both items is only seen in that context. The example here is spick and span but others include:
    to and fro
    harum-scarum
    helter-skelter
    hem and haw
    nip and tuck
etc.

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Duplex expressions

Most strong collocations are perfectly easily understood by understanding the individual lexemes.
For example, there is no difficulty at all in understanding
    counterfeit money
    bank account
    garden party
    make a choice
    deeply regret
    wholly unacceptable
and thousands more by understanding the words in them, even when, as in the fourth case, the verb carries little real meaning.

There are, however, some collocations which may be literal in meaning in one sense and used figuratively in another. For example:
    a fine-tooth comb
may be reference to a particular type of comb which has small gaps between the teeth. However, when we say:
    They went through the accounts with a fine-tooth comb
we are referring to very careful examination and not to a comb at all. Equally,
    a slippery slope
may simply refer to an ice-like surface on an incline but in something like:
    She's on a slippery slope to being dismissed
we are referring to the fact that small actions or omissions can result in large, negative consequences.
Furthermore, some idiomatic expressions which look like simple collocations are, in fact, only used figuratively so, for example:
    He's a big noise in the army
refers to someone who is important and influential but the words big and noise do not elsewhere form a natural collocation (the preferred adjectives being, loud, huge, deafening etc.).

Macis and Schmitt (2017) estimate that a quarter of strong collocations have figurative (i.e., idiomatic) meanings and are, therefore, more sensibly dealt with in the classroom as idioms which should be learned and produced as single lexical chunks.
Looking out for collocations which are, in fact, being used figuratively and idiomatically may pay dividends in terms of teaching and remembering lexis.

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Classifying idiomatic language

We can use the concepts of opacity and fixedness to try our hand at classifying the range of idioms in English. Such a classification ignores form for the most part and that we shall deal with shortly but it does give us a way of making sense of large amounts of data. It also, unfortunately, means that categories have blurred edges but that is what always happens when we try to draw lines on a continuum. This will also give us the opportunity to add a few more examples of the kinds of idioms we have discussed so far.
For teaching purposes, the following categories have some utility because it allows us to focus our learners on idiomatic language which shares definable characteristics rather than just showering them with random examples of idioms. Later, we will suggest other ways that idioms can be classified for teaching purposes which rely more heavily on meaning and conceptual hooks for their effect.
It is also possible to add to the categories we will use here by inventing new intermediate ones to extend the possibilities but blurred edges will always be with us.
Whether you choose to use fixedness or opacity as the measuring stick for these categories is entirely up to you. Either approach will make sense in the classroom. The first categories in the first two sections of what follows are the same.

Fixed idioms

Fixedness does not always refer to an absolute quality because function words (particularly determiners) and tense forms may be altered to suit the context. So, for example, we can have:
    keep an / your / my / his eye on
in which the lexical items (keep, eye) cannot be altered and even the preposition is fixed. The determiner can vary however and so can the tense. It is, therefore, rare for any of the lexical constituents to be changed (and even number and mood are frequently fixed). For example:
    bells and whistles
is always plural
    storm in a teacup
is always singular and the determiner is also fixed.
    make head or tail of
is always negative.

Fixed and opaque
These idioms usually constitute single lexemes and are learnable as language chunks.
These types are also characterised by having a high level of non-compositionality. Even with context and co-text, it is often impossible to work out the meaning despite knowing all the words which make up the expression.
Examples are:
    a can of worms (a complex and difficult situation or set of things)
    the black sheep of the family (a family member who is considered to deviate from the norm)
    up the garden path (a misleading direction)
Fixed and semi-opaque
While maintaining a high level of fixedness, these idioms may be accessible to comprehension providing one is alert to metaphorical and figurative uses of lexemes. Context and co-text will often help.
Examples are:
    as different as chalk and cheese (completely different)
    neither here nor there (irrelevant)
    making a mountain out of a molehill (exaggerating an issue)
    bend over backwards (go to great lengths to be helpful)
Fixed and transparent
Here we verge on strong collocation rather than idiomaticity per se. These expressions are fixed but if you know the meaning of the elements within them, the meaning can be easily surmised so they are virtually transparent.
Examples are:
    set in stone (a fixed rule)
    love at first sight (instant adoration)
    a bull in a china shop (being careless of the damage one's actions may cause)

Opaque idioms

Opaque and fixed
This is essentially the same category as 1. above because we are identifying the extreme ends of the clines. As above, these expressions usually operate as single lexemes and can be learned and used as prefabricated chunks.
    feeling under the weather (ill)
    the back of beyond (remote and isolated)
    pass the buck (transfer responsibility or guilt to another)
Opaque and semi-fixed
These are opaque but some variation is possible. They are, however, not accessible for meaning without a great deal of guesswork and use of co-text and context. Even then, comprehension is not assured.
    throw in the towel / sponge (resign or surrender)
    take the mickey / piss / a rise (ridicule)
    have a bash / go / stab / shot (try)
Opaque and variable
Here again, we wander into the realm of simple collocation or delexicalisation. The opacity stems from the fact that the verb is empty of any obvious meaning so it can only be processed by understanding the noun and even then may remain opaque.
    make a promise / a friend / a difference
    pay a visit / a complement / one's respects
    catch a cold / sight of / fire / a train
    lose heart / confidence / the plot / the thread

Metaphorical or figurative uses

Verbs
This differs from delexicalised use of verbs insofar as the verb retains its meaning but it has been extended to the point at which the meaning is no longer transparent although some imagination may help to unpack the significance.
Examples are:
    spend time
    close a deal
    pencil someone in
    crash a party
    fall asleep
    bear a hand
    drop a hint
Nouns
These are often nouns being used outside the field in which they normally operate.
    a team player
    a point of view
    a short head
    a hair's breadth
    a bird's eye view
    a rat's nest
    the end of one's tether
Adjectives and adverbs
Colour adjectives are frequently used figuratively and cultural influences come into play. Other adjectives and adverbs referring to perceptions are also often used figuratively. Most are sometimes considered very strong collocations but, because of the metaphorical use, we prefer to classify them as idioms. They are not duplex forms because there is no literal sense.
example are:
    a black look
    a white lie
    a warm welcome
    a hot reception
    hotly contested
    coolly received
    bitterly contested
    flatly refuse

We can summarise all this like this:
classify

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The relationship between fixedness and opacity

There is a tendency for these two characteristics to rise and fall together. In other words, the more fixed and inflexible the expression, the more likely it is to be opaque in meaning and vice versa.

We can find low fixedness with some expressions but they are likely to be quite literal in meaning. For example, as old as ... can be followed by a number of expressions (God, the hills, Noah etc.) but opacity is also low.
Similarly, strong collocations such as a pronounced accent are not firmly fixed (we can have strong, broad etc. as the adjective with roughly the same meaning) but they are usually easy to understand (if not to learn). Such expressions have low fixedness and also low opacity.

On the other hand, an expression such as off one's rocker has quite high fixedness (there's only one conventional alternative to rocker, trolley) and it is also quite high in opacity. Extreme cases of fixedness are also, often, extreme cases of opacity. Expressions such as let the cat out of the bag are both opaque and fixed as are binomials such as helter-skelter.

There are, nevertheless, instances, especially with binomials, of low opacity and high fixedness. In other words, they always occur together and in the same order but are straightforward to understand. Examples are:
    here and there
    hand in hand
    dead and buried
etc.

The moral?
Whenever we find a highly opaque expression, the way to bet is that it is also firmly fixed. The reverse is not always true: firmly fixed expressions are not always opaque..
Here's a graphical representation of that. Before teaching idioms, it is worth 5 minutes of the planning time to consider where in the matrix the target language items fall.
matrix

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One-word idioms

Idioms are usually understood to be phrases or clauses which cannot be immediately understood by retrieving the meaning of the words in them. However, many words, especially verbs, are or can be used metaphorically and qualify as idioms because they are (obviously) fixed with synonyms rarely having the same effect and they are often quite opaque in meaning.
Some examples are:
    The show bombed
    I was bombarded with questions
    The question threw me
    They sacked him
    You have lost me
    It's a pig
    My printer died
etc.
None of these examples will mean the same when synonyms or near synonyms are used so, for example:
    The show shelled
    I was blasted with questions
    The question flung me
    They plundered him
    You have misplaced me
    It's a hog
    My printer perished
are all either incomprehensible or extremely unnatural.

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Binomials and trinomials

Because these are so common in English, they merit a short section to themselves. Many of these items are worth teaching as single lexemes because they are handy language chunks, they are extremely common and they are not easily paraphrased.
There are some general characteristics of binomial expressions:

Form:
They consist of two lexical items belonging to the same word class so they are, noun + noun, verb + verb, adjective + adjective, adverb + adverb. Examples of the four main sorts are:
Noun + Noun:
    He has lived here man and boy
Verb + Verb
    You can take it or leave it
Adjective + Adjective
    And that's the truth, pure and simple
Adverb + Adverb
    I will do it sooner or later
(Rarely, the two items are not of the same word class but follow similar structural forms so, for example:
    We were home and dry
in which home is an adverb and dry an adjective but both are joined to the subject by the copular verb. That is probably not something with which to trouble your learners.)
Meaning:
Some are literal (apples and oranges etc.), some are figurative (the chicken or the egg etc.) and some are wholly opaque (milk and honey, fiddle-faddle etc.).
Rarity:
Some, such as helter-skelter, super-duper, to and from, hither and yon etc., contain words found nowhere else or retained from archaic forms.
Concord:
When two nouns are joined, the resulting expression is often singular, e.g.:
    Fire and brimstone is all he shouts about
    Thunder and lightning is on its way
but, if the nouns are already plural, that is not the case:
    His eyes and ears are everywhere
    The stars and stripes are flying over The White House
Ordering:
The order of the items is usually fixed although with some, reversal has no effect. We can have:
    She worked day and night
and
    She worked night and day
We allow
    I'll do it sooner or later
but not
    *I'll do it later or sooner
Inflections:
The tenses, verbs forms, adjective inflexions and numbers of items are normally retained in both constituents so we get, for example:
    It's done and dusted
    That's bigger and better
    It comes with many bells and whistles
    I'll name and shame him
    There will be some naming and shaming
    They were named and shamed
etc. and:
    *for all intent and purpose
    *It's time to cut and be running
are not encountered.
Coordination:
The items are frequently joined with the coordinator and but there are other possibilities including: but, or, either ... or, neither ... nor, to, after, by, in.
Prosody:
Often the items rhyme or are, more often, alliterative as in, e.g.:
    make or break
    high and dry
    house and home
    do or die
etc.
In some, a phenomenon called assonance is discernible so for example, a stressed vowel will be the same in both items or a consonant duplicated with different vowels as in
    harum scarum
or
    tittle-tattle.
Collocation:
Because binomials operate as single lexemes, they are subject to the same collocational forces as all other lexemes so, for example:
    high and dry collocates strongly with the verb leave
    high and low collocates with verbs such as look (for), search, hunt and seek
    dead and buried collocates with nouns such as ideas, proposals, suggestions, schemes and plans
and so on.
Effect:
Binomials often intensify, especially reduplicative ones, so, e.g.:
    She went from strength to strength
    They asked again and again
Ones in which the two items are synonyms have the same effect:
    He's at my beck and call
    The truth is pure and simple
and, perversely, antonym pairs also intensify:
    We searched high and low, in and out, in each and every part of the house
The intensification is either echoic (repeating the word) as in the first two examples or a function of synonymy or antonymy as in the other examples.

Here's a selection.
Fuller lists with some doubtful inclusions are available via a web search.

joined with coordinators (and or or/neither ... nor etc.)

above and beyond
airs and graces
alive and kicking
all or nothing
an arm and a leg
apples and oranges
assault and battery
back and forth
ball and chain
beck and call
beer and skittles
bells and whistles
for better or worse
betwixt and between
bits and bobs
bow and arrow
by and large
cat and mouse
the chicken or the egg
cut and dried
cut and run
day or night
dead and buried
dead or alive
divide and conquer
do or die
down and out
each and every
eyes and ears
far and wide
guys and girls
horse and carriage
intents and purposes
kill or cure

kill or be killed
knife and fork
law and order
love nor money
lo and behold
loud and clear
make or break
man and boy
milk and honey
more or less
neat and tidy
nip and tuck
nook and cranny
fast and loose
fingers and thumbs
fire and brimstone
first and foremost
forever and a day
free and clear
fight or flight
(neither) fish nor fowl
fun and games
(come) hell or high water
(neither) here nor there
hit or miss
hale and hearty
hard and fast
hearts and minds
here and now
high and dry
high and low
home and dry
hope and pray
nuts and bolts

odds and ends
pure and simple
pepper and salt (hair colour)
rags to riches
rain or shine
research and development
room and board
sink or swim
sooner or later
take it or leave it
salt and pepper (seasoning)
seek and destroy
short and / but sweet
sick and tired
slash and burn
smash and grab
snakes and ladders
stand and deliver
supply and demand
sweetness and light
tables and chairs
tar and feather
tea and crumpets
thunder and lightning
time after time
to and fro
toil and trouble
tooth and nail
touch and go
trial and error
up and about
vim and vigour
wait and see
wine and roses

with reduplication
The term reduplicate is a slight misnomer because the words are duplicated, not reduplicated.

again and again
all in all
around and around
arm in arm
back to back
bit by bit
bumper to bumper
by and by
cheek to cheek
closer and closer
coast to coast
day to/ by day
elbow to elbow
end to end
dog eat dog
from ear to ear
an eye for an eye

eye to eye
face to face
hand in hand
head to head
heart to heart
higher and higher
horror of horrors
less and less
little by little
lower and lower
man to man
more and more
mouth to mouth
neck and neck
never say never
nose to nose
on and on

one by one
out and out
over and over
round and round
shoulder to shoulder
side to side
step by step
strength to strength
through and through
time after time
(from) time to time
two by two
toe to toe
up and up
wall to wall
for weeks and weeks
woman to woman

with rhymes or similar sounds
These are often considered a subset of reduplicate phrases but exactly where the border between a reduplicative and these examples lies is not always easy to determine. Parts of the words are clearly duplicated and the words often rhyme, a phenomenon encapsulated in the alternative name, ricochet words.
Many of these items contain words unused elsewhere in the English lexicon and they exhibit both very high noncompositionality as well as fixedness so they are idioms par excellence.

belt and braces
box and cox
chalk and talk
chit-chat
claptrap
dilly-dally
ding-dong
double trouble
even Stevens
fender-bender
fiddle-faddle
flim-flam
flip-flop
hanky-panky
harum-scarum
helter-skelter
†higgledy-piggledy
high and dry

hire and fire
hither and thither
hocus-pocus
hodge-podge
hoity-toity
horses for courses
hubble-bubble
huff and puff
hurly-burly
hustle and bustle
meet and greet
mish-mash
namby-pamby
name and shame
near and dear
knickknacks
nitty-gritty
odds and sods

out and about
pell-mell
ping-pong
pitter-patter
razzle-dazzle
riff-raff
roly-poly
shillyshally
time and tide
tip-top
tittle-tattle
town and gown
use it or lose it
wear and tear
willy-nilly
wine and dine
wishy-washy
yea or nay

† there is disagreement about the spelling of this item so we may see, higgelty-piggelty, higgeldy-piggleldy as well.
Hyphenation (see below) is also variable and idiosyncratic with some items so we also see flimflam, shilly-shally, mishmash, chitchat, knick-knacks, dingdong etc.

ordering the items
There are, interestingly for some, rules for which item is placed first when the vowel sounds differ and they have to do with what is called ablaut reduplication. Ablaut describes the alternation in vowels that we see in, e.g., sing, sang, sung or ring, rang, rung and so on.
Although there are very rare exceptions, the rule is that the first item contains a higher vowel than the second. That is to say, the tongue is higher in the mouth when the vowel in the first item is produced than in the second.
High vowels in English include: /ɪ/ as in kid, /iː/ as in heel, /ʊ/ as in foot and /uː/ as in shoe.
Low vowels include /æ/ as in hat, /ɒ/ as in wash, /ʌ/ as in cup, /ɔː/ as in taught and /ɑː/ as in car.
So it is that we get shilly-shally, not shally-shilly, ping pong, not pong ping, chit-chat not chat-chit, ding-dong not dong ding, flim-flam and flip-flop not flam-flim and flop-flip and so on.
It also explains why most speakers will say this and that not that and this, knife and fork, not fork and knife and so on.
This phenomenon is rarer for trinomials in the list below but is evident in, e.g., cool, calm and collected, eat, drink and be merry and this, that and the other.
This appears to be non-language specific, incidentally.

Some quite common words in English are derived by reducing rhyming or similar-sounding binomials so, e.g.:
    patter is a reduction of pat-pat (to hit gently).
    blabber is a reduction of blab-blab
    paddle is a reduction of pad-pad
and so on.
The technical term for this phenomenon is that the word is a frequentative.

trinomials

There are fewer of these and they almost always employ and as the coordinator. Examples include:

beg, borrow or steal
blood, sweat and tears
cool, calm and collected
eat, drink and be merry
ear, nose and throat
gold, silver and bronze
guns, germs and steel
healthy, wealthy and wise
here, there and everywhere

hook, line and sinker
hop, skip and jump
judge, jury and executioner
left, right and centre
lights, music, action
lock, stock and barrel
nasty, brutish and short
planes, trains and automobiles
ready, willing and able

reading, writing and arithmetic
red, white and blue
sex, drugs and rock 'n' roll
tall, dark and handsome
Tom, Dick and / or Harry
shake, rattle and roll
this, that and the other
way, shape or form
win, lose or draw

Comma use is shown here without the Oxford comma (after and) as above but that is often inserted.

hyphenation

Many binomials, especially those without a connecting conjunction, are conventionally hyphenated so we get helter-skelter, willy-nilly, harum-scarum and so on.
Others are only hyphenated when they are used adjectivally so we get, for example:
   It's a question of law and order
    The price is subject to the influence of supply and demand
etc., because these are being used as nouns, but we have:
This is a law-and-order issue
    It's a supply-and-demand influence
etc., because these are adjectival uses.
Trinomials exhibit the same phenomenon.

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Frozen or fixed similes

Similes explicitly compare two items, usually, in English, with the as ... as formulation.
A number of these constitute a kind of idiom although in almost all cases they are a) fixed and b) often (but not always) quite literal and transparent in meaning. To extract the meaning, one has usually only to understand the first item and then understand that the idiom emphasises its strength.
They include items such as:

as blind as a bat
as black as night
as cheap as chips
as clean as a whistle
as cool as a cucumber
as dead as a doornail

as deaf as a post
as fit as a fiddle
as flat as a pancake
as free as a bird
as fresh as a daisy
as good as gold

as hard as nails
as hard as (a) stone
as keen as mustard
as large as life
as light as a feather
as mad as a hatter

as old as the hills
as plain as day
as poor as a church mouse
as quiet as a mouse
as regular as clockwork
as right as rain

as safe as houses
as sharp as a knife
as sick as a dog / cat
as strong as an ox
as stubborn as a mule
as thin as a rake

etc.
Such expressions, too, are often alliterative.
The main elements are usually given equal stress.

A second form for these fixed expressions employs the like preposition. These are not adjectival so, often, it is a noun being compared to another or a verb being used figuratively as in

Nouns	Verbs	Modified nouns
ears like a bat eyes like a hawk a face like a brick wall a face like a torn balloon a face like thunder a grip like a vice a hand like a bunch of bananas a hand like a claw legs like tree trunks a look like thunder a mouth like a trap skin like a rhino a voice like a foghorn like water off a duck's back	cry like a baby drink like a fish eat like a bird eat like a horse fight like cat and dog fit like a glove go like lightning go like the wind run like clockwork sell like hot cakes sleep like a baby smoke like a chimney spin like a top stick out like a sore thumb work like a Trojan	be like a child in a sweet shop be like chalk and cheese be like a sight for sore eyes be like a dog in a manger be like a fish out of water be like a breath of fresh air be like a bull in a china shop be like a dog with two tails be like two peas in a pod be like a moth to a flame be like the cat that got the cream be like a headless chicken be like watching paint dry be like shooting fish in a barrel

They are almost totally confined to informal speech and writing. Such clichés are often disparaged in more formal texts.

Similes can also be created in English with the use of neither the as ... as nor the like formulation. Here's a short list of what is meant:
    crystal clear (=as clear as crystal)
    feather light (=as light as a feather)
    grass green (=as green as grass)
    ice cold (=as cold as ice)
   paper thin (=as thin as paper)
    razor sharp (=as sharp as a razor)
   sky high (=as high as the sky)
    stone cold (=as cold as a stone)
etc.
These formulations are rarer.

Such expressions are also sometime abbreviated with -like as the adjective-forming suffix to signal that we are referring to a simile so we also encounter:
    claw-like hand
    ghost-like face
    glove-like fit
    vice-like grip
but such expressions are unpredictably acceptable and cannot be formed from the majority of like similes.

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Style and register

Most idiomatic language is stylistically informal and inappropriate in a number of situations. Idioms are used extensively in informal speech and writing (especially in newspapers), however, so a knowledge of common ones is very helpful for learners of English. Unfortunately, there are, by some estimates, 25,000 of them in English.
In more formal contexts, idioms will often be avoided so we are unlikely to find, for example:
    Buckingham Palace announced that the Queen is under the weather
    The government negotiators are reluctant to open a can of worms, said the White House spokesman.
etc.
Learners of the language can be tempted to overuse idiomatic language in situations where it is not appropriate or they can get the meaning just slightly wrong and produce, e.g.:
    *I'll do it willy-nilly
    *I'm down in the dumps with her
    *The government isn't cutting the mustard for it
In common with many idiomatic expressions, bi- and trinomials are often informal and common in spoken language.
A few, however, such as first and foremost, more and more, intents and purposes and others are encountered in formal writing and some are confined to specific registers such as economics (supply and demand), the law (aid and abet), education (reading, writing and arithmetic) or engineering, politics and commerce (research and development, wear and tear, trial and error, ways and means).
They are, to some extent, clichés and accordingly much used in journalese.

The same considerations of grammar and form apply here as they do in the teaching of any lexis.
It is important to make sure, then, that idiom presentation is set in an appropriate context (both style and register) and that word class is considered along with aspects of transitivity, complementation and so on.

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Pronunciation

Most binomials are evenly stressed with the coordinating item being weakened. We get, therefore:
   name and shame /ˈneɪm.n̩.ˈʃeɪm/
    fun and games /ˈfʌn.n̩.ˈɡeɪmz/
    tittle tattle /ˈtɪt.l̩.ˈtæt.l̩/
    shoulder to shoulder /ˈʃəʊl.də.tə.ˈʃəʊl.də/
A few may operate like compound nouns and take the stress on the first element, e.g.:
    near and dear (/ˈnɪər.n̩.dɪə/)
but stressing both components equally is more common.
Trinomials are lists but, because they are processed as single units, the normal list intonation, with rises after each item and a fall at the end, is compromised. We may get, therefore, for example:
game, set and match
pronounced as
/ˈɡeɪmset.n̩.ˈmætʃ/
rather than
/ˈɡeɪm.ˈset.ənd.ˈmætʃ/
with the normal pause after the first item and equal stress on all the items.
More commonly, the stress on each item is retained but the intonation does not rise as it would in a list of less connected items:
beg, borrow and steal /ˈbeɡ.ˈbɒ.rəʊ.n̩.ˈstiːl/
Many idioms operate as single lexemes and only one tone unit is discernible so we have, e.g.:
I wouldn't be caught dead
/ˈaɪ.wʊdnt.bi.kɔːt.ˈded/
with only two stressed syllables.
Compare:
I wouldn't do that if I were you:
/ˈaɪ.wʊd.də.ˈðət.ɪf.ˈaɪ.wə.ˈju/
with four stresses.
Because of their single lexeme nature, idioms are spoken rapidly as one chunk so syllable reduction and weak forms are common. E.g.:
She's bitten off more than she can chew
is often pronounced as
/ʃiz.ˈbɪt.n̩.ə.mɔː.ðən.ʃi.kn̩.tʃuː/
with two syllabic consonants and weak forms (/ə/, /i/). See, too, the pronunciation of and in the binomial examples which is often reduced to a single syllabic consonant (/n̩/).

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Teaching idiomatic language

Too often, in coursebooks and study guides, idioms and idiomatic language are relegated to peripheral 'Useful phrases' boxes and then ignored. That's a great pity as it is almost impossible to become fluent in English without acquiring a fair number of idiomatic expressions. In fact:

Most students are very interested in learning idiomatic language. They recognize it as an area in which they have difficulties, and appreciate systematic instruction.
(Irujo, 1986: 242)

There's nothing mysterious about this. We have to make the same judgements that we make when teaching lexis of any sort. In other words, we must consider appropriacy and style, range, learnability, frequency and so on. For more, see the guide to teaching lexis, linked below.

Difficulties

Idioms present some serious difficulties for learners and that is one reason that they have fallen out of fashion of late.

Idioms date quite rapidly.
It may be the case that native speakers of English used to exclaim
    Ye Gods and little fishes!
when astonished by something but to do so now would make the speaker sound very odd and old fashioned.
The popular example of dating is the expression
    raining cats and dogs
which almost no native speaker would use but which is often heard from learners of English. Other examples of dated idioms to avoid include:
    to be in seventh heaven
    to keep up with the Joneses
    the gift of the gab
    to be dead beat
    to take a rise out of someone
and there are many more with which it is unwise to burden learners.
Idioms require learning and reproducing as a string of words exactly.
Some idioms are quite long strings and impose something of a strain on the memory. For example:
    choose the lesser of the two evils
    give someone the benefit of the doubt
    cross that bridge when you come to it
are all quite long and hard to remember as language chunks.
Fixedness and naturalness.
Getting even one word wrong in an idiom can often result in the expression sounding absurd and unnatural, although it will rarely be misunderstood. For example:
    *That's a sour pill to swallow
    *I wash my hands from it
    *Speak of a devil
    *It's not rocket physics
are all wrong by one word and even missing or misusing an article can make the speaker sound foreign or amusing.
Some idioms are stand-alone expressions but some can only be used embedded in co-text. For example:
    Get your act together
    Better late than never
    So far so good
    You can say that again
are all routinely used as isolated phrases (although they can be embedded in other language).
However:
    the best of both worlds
    miss the boat
    beat around the bush
    on the ball
    in the mire
are only used when they are embedded in other language and cannot stand alone.

Classification and selection for teaching

One can, of course, teach idioms only as and when they arise in texts used in the classroom or in response to learner enquiries. That makes some sense because it avoids problematising the issue and demystifies a complicated area.
However, especially at more advanced levels, many learners appreciate that they can only sound truly natural in English if they are able to use a reasonably wide range of idiomatic expressions. They will often, therefore, appreciate and be motivated by lessons which focus specifically on idioms and idiomatic language.
The teacher's problem is how to select, group and present the most common ones. There are three main ways:

A shared conceptual category
It has been noted that some words in English feature in idioms more than most and provide a jumping-off point as well as a conceptual hook to help people remember the expressions. For example:

colours	verbs / nouns	body parts	animals	nature
black and blue black and white black looks black market black out to be in the black blue movies browned off be green with envy green politics give the green light have green fingers men in grey suits in the pink purple prose a red-letter day catch red handed red blooded red carpet see red red tape to be in the red scarlet woman yellow bellied	bark ... worse than ... bite big bang bite off more than you can chew bite the bullet bite your tongue buy the farm catch / get a name catch a programme catch (+ transport) catch it (be punished) catch someone (meet) there's a catch fall foul of something go the extra mile grab a quick bite kick into the long grass like it or lump it make the grade nuclear option open your heart open and shut pay the piper raise Cain run it up the flagpole run a risk throw a party throw a fit throw a wobbly throw in the towel / sponge throw someone (puzzle or confuse) throw the toys out of the pram throw your hat in the ring	cost an arm and a leg play by ear elbow in keep an eye on see eye to eye give your eye teeth for a finger in many pies make a fist of best foot forward foot the bill footloose and fancy free face the music face to face lose face get in someone's face face up to put your foot in it have / get cold feet find one's feet a head start put your head in the sand let your hair down head for home head in the clouds head over heels dig your heels in over my head be neck and neck bend the knee have a knees up put your nose in shoulder responsibility by the skin of your teeth have a sweet tooth get your teeth into something tooth and nail toe the line	bear / bull market bear with a sore head beaver away eager beaver let the cat out of the bag headless chicken chicken livered cow eyes a milch cow crocodile tears crow about dog tired let sleeping dogs lie do the donkey work elephant in the room ferret around a fish out of water a cold fish a fly on the wall a fly in the ointment wouldn't hurt a fly get someone’s goat a wild goose chase hare around flog a dead horse back the wrong horse up with the lark the lion's share lounge lizard pig out rabbit in the headlights smell a rat rat on snake in the grass squirrel away pearls before swine keep the wolf from the door a wolf in sheep’s clothing worm your way in	black hole blizzard of paperwork a breeze the cliff edge an Everest to climb have a feather in your cap flood of applications fog of confusion forest of legs gale of laughter light years away lightning quick mists of time mountain of work nip something in the bud ocean of tears quantum leap ripple of applause river of demonstrators sea of troubles storm of protest stream of invective thunderous footsteps vale of tears wave of protest wet behind the ears wet blanket

A shared source
We saw above that many idioms derive from professions and activities and this too can be a way of helping the memory. For example:

sports and leisure activities	warfare	commerce and business	seafaring
the ball is in your court off his own bat be blinkered push the boat out paddle your own canoe mark someone’s card jump the gun be for the high jump jump through hoops hold your horses a marathon not a sprint to give a free rein to keep on a tight rein a front runner be in the saddle shoot your mouth off call the shots skate around the problem snookered to spur on the home stretch a sticky wicket down to the wire	attack a problem bombard with questions have a blitz damage limitation dig in pull up the drawbridge wave a white flag the big guns to steal a march get your head over the parapet take no prisoners rank and file beat a retreat scattershot lay siege to snipe go over the top torpedo an idea circle the wagons a war of nerves a war of words to be in the wars	a blank cheque the bottom line break the bank close a deal corner the market do a deal do the business get down to business have a monopoly insider information open an account pay dividends pay over the odds return with interest rig the market sell short sign on the dotted line stakeholders trade in	be taken aback batten down the hatches brass monkeys a broadside choppy water clear the decks all hands on deck fly the flag hull up hit a rock / reef keep a lookout ride out a storm know / learn the ropes sail before the wind set sail a shot across the bows show your true colours smooth sailing get a head of steam swing the lead toe the line in the wake of

A shared function
Idioms can be classified by the sorts of things they refer to and this is also a useful way of helping the memory. For example:

describing personality	complementing	complaining
a pain in the neck a dark horse a bright spark a wet blanket a rough diamond the life and soul a wallflower a shrinking violet	She's a diamond You're a trooper You're a brick He's a class act It's worth an Oscar Take a bow You've saved my life You're a star	breathe down someone's neck Pull your weight Get off my back pay through the nose argue the toss have a bone to pick make a mountain out of a molehill a backseat driver

Lists

A web search will reveal some very long lists of idioms which you may be tempted to teach to your long-suffering learners. There is nothing wrong with accessing such materials providing:

you are aware that many lists are collected by people who do not really understand the difference between an idiom, a proverb and a strong collocation.
you are alert to issues of fixedness and opacity and react accordingly making a principled selection
you know a little about how learners may be helped to remember items by linking them to conceptual categories or embedding them in a memorable setting or text

With all that in mind, the following is a selected list of around 380 common idioms which attempts to exclude items which are not actually idioms at all, ones which are rare or ones confined to certain registers only. We have also excluded any we didn't know and those which, while classifiable as idioms, are wholly transparent in meaning.

above and beyond
above board
accident of birth
accident waiting to happen
ace in the hole
ace up one’s sleeve
Achilles’ heel
acid test
add insult to injury
albatross around one’s neck
alive and kicking
all and sundry
all bets are off
all ears
all hands on deck
all over bar the shouting
all the tea in china
all thumbs
an axe to grind
an early bird
an eye for an eye
any port in a storm
apple of one’s eye
arm and a leg
armed to the teeth
as fit as a fiddle
as pale as a ghost
as poor as a church mouse
asleep at the wheel
at the drop of a hat
at the eleventh hour
at the end of one’s tether
at one’s wit’s end
back of beyond
back to square one
back to the drawing board
bad apple
bag of tricks
ball in your court
bang one’s head against a brick wall
bark up the wrong tree
basket case
batten down the hatches
beat a dead horse
beat around the bush
bed of roses
bee's knees
behind the scenes
behind the times
bells and whistles
best thing since sliced bread
best of both worlds
between a rock and a hard place
between the devil and the deep blue sea
between two stools
beyond the pale
bird brain
bird’s-eye view
bite off more than you can chew
bite the bullet
bite the dust
bitter pill to swallow
black sheep
blow hot and cold
bolt from the blue
bone dry
break the ice
brush / sweep something under the carpet
bump in the road
burn the candle at both ends
burn the midnight oil
bury the hatchet
by the skin of one’s teeth
call a spade a spade
call it a day
call the shots
carrot-and-stick
carry the can
Catch-22
caught red-handed
champ at the bit
chase rainbows
cherry-pick
chew the fat
chickens coming home to roost
child’s play
chink in someone's armour
chip off the old block
claim to fame
clear the air
close, but no cigar
cold shoulder
come clean
come hell or high water
come rain or shine
cool as a cucumber
couch potato
cry over spilt milk
cry wolf
cut corners
cut the mustard
dance to someone’s tune
dark horse
dead as the dodo
dead heat
dead ringer
devil’s advocate
dig one's heels in
dime a dozen
dodge a bullet
dog in the manger
double-edged sword
down in the dumps
drag one’s feet
draw a blank
drive a hard bargain
drive someone up the wall
drop a line
dry run
dyed-in-the-wool
eagle-eyed
early bird
eat humble pie
eat your heart out
elephant in the room
eleventh hour
every man and his dog
every man for himself
face the music
fall prey to
fat cat
father figure
feather in one’s cap
feather one’s own nest
fed up with
fell off the back of a lorry
fight fire with fire
fight like cat and dog
find your feet
fish out of water
flash in the pan
fly in the ointment
fly off the handle
food for thought
Freudian slip
from pillar to post

from scratch
full of the joys of spring
get a word in edgewise
get carried away
get off scot free
get one’s hands dirty
get someone’s goat
get the ball rolling
get the picture
get the sack
get to grips with
get wind of
give someone a piece of your mind
give someone a run for their money
give someone the cold shoulder
give the green light
go bananas
go berserk
go down in flames
go off half-cocked
go off the deep end
go out on a limb
go pear-shaped
go the extra mile
go to the dogs
go viral
go with the flow
grasp at straws
grasp the nettle
grease the wheels
grind one’s teeth
guilty pleasure
guinea pig
hair of the dog
hands down
hanging by a thread
have (one’s) head in the clouds
have a bone to pick with someone
have a chip on one’s shoulder
have a lot on one’s plate
have a nose for
have a whale of a time
have an ace up one’s sleeve
have bigger fish to fry
have egg on your face
have one’s cake and eat it
have something in the bag
have your say
head and shoulders above
head over heels
head start
hear on the grapevine
heart and soul
hell for leather
hit a wall
hit the ceiling
hit the ground running
hit the hay
hit the nail on the head
hit the road
hit the roof
hit the sack / hay
hit the spot
Hobson's choice
hold one’s peace
hot on the heels of
hot potato
in a heartbeat
in a jam
in a nutshell
in a rut
in broad daylight
in clover
in hot water
in one fell swoop
in the blink of an eye
in the dark
in the driver’s seat
in the limelight / spotlight
in the long run
in the nick of time
in the pipeline
in the same boat
in the works
it’s not rocket science
itchy feet
Jack of all trades
join the club
jump on the bandwagon
jump ship
jump the gun
jump through hoops
just what the doctor ordered
just the ticket
keep a stiff upper lip
keep an eye peeled
keep it under your hat
keep your powder dry
kick it into the long grass
kick the bucket
kick the habit
kill two birds with one stone
kiss and make up
kith and kin
know something like the back of one’s hand
learn / know the ropes
leave someone in the lurch
let sleeping dogs lie
let the cat out of the bag
like ... (see similes, above)
lion’s share
look a gift horse in the mouth
lose the thread
low-hanging fruit
make a mountain out of a molehill
make ends meet
make one’s mark
meeting of the minds
miss the boat
move heaven and earth
much of a muchness
music to one’s ears
neck and neck
nip something in the bud
no rhyme or reason to
nothing to write home about
off the hook
off your trolley / rocker
once in a blue moon
paddle one’s own canoe
pain in the neck
par for the course
pass the buck
pass with flying colours
pencil something in
penny-pinching
pie in the sky
piece of cake
pipe dream
play it by ear
pop your clogs
pot calling the kettle black
peaching to the choir
pull someone's leg
pull strings
pushing up the daisies

push the envelope
put the cart before the horse
put the cat among the pigeons
put your foot down
queer the pitch
quick as a flash
quick-and-dirty
quote unquote
race against time
rain cats and dogs
raise the bar
raise the roof
rank and file
read between the lines
read the riot act
red herring
red tape
reinvent the wheel
right as rain
right under one’s nose
right-hand man
ring a bell
rock bottom
rock the boat
rub something in
rubber-stamp
rule of thumb
run out of steam
sacred cow
sandwich short of a picnic
second wind
see eye to eye
sell like hotcakes
set in stone
set the Thames on fire
sharp as a tack
shoot the breeze
shooting fish in a barrel
show one’s true colours
sick and tired of
sight for sore eyes
silver bullet
sink or swim
sitting duck
sitting pretty
six of one, half a dozen of the other
sleep like a baby
slippery slope
small beer
smell a rat
smoking gun
snake oil
sore point
sour grapes
spick and span
spill the beans
spin a yarn
square the circle
stab in the dark
stab someone in the back
stand on one’s own two feet
stand one’s ground
steal someone’s thunder
stem the tide
step up to the plate
storm in a teacup
strike a chord
sweep under the carpet / rug
swim against / with the tide
sword of Damocles
take a gander
take a rain check
take it easy
take it or leave it
take something with a pinch of salt
take the biscuit
take the bull by the horns
take the edge off
take the mickey / piss
take the wind out of someone’s sails
take with a grain of salt
take your medicine
taste of your own medicine
teach an old dog new tricks
tear one’s hair out
ten / two a penny
test the waters
that ship has sailed
there’s no such thing as a free lunch
thin on the ground
think outside the box / envelope
through the grapevine
through thick and thin
throw a spanner in the works
throw caution to the wind
throw down the gauntlet
throw in the towel / sponge
throw out the baby with the bathwater
thumb your nose at
tight-lipped
till the cows come home
time is money
tip of the iceberg
toe the line
tongue-in-cheek
too many to shake a stick at
touch base
tread water
turn a blind eye to
turn the corner
turn the tables
twist the knife
make a U-turn
under / below the radar
under someone's thumb
under the weather
until the cows come home
until you’re blue in the face
up for grabs
up in the air
up to scratch
upset the apple cart
use one’s head
vicious circle
virgin territory
vote with one’s feet
waiting in the wings
walk on eggshells
water under the bridge
wet behind the ears
wet blanket
what goes around comes around
when the chips are down
wild goose chase
window dressing
window shop
witch hunt
work one’s fingers to the bone
writing on the wall
year in, year out
you can say that again
you can’t judge a book by its cover
young at heart
your guess is as good as mine
yours truly
zero in on something

If you would like all those lists as a single PDF document, it's available here and from the links at the end.

Clearly, such a list is only useful for planning purposes and as an aide memoire. Please do not be tempted to give it to your learners to commit to memory.
One way of approaching the area which has found some success is to alert the learners that every lesson will include a new idiom from this list. Their job is to try to identify it when it appears in the materials or in something you say and point this out to the class.
In this way, an enjoyable and simple challenge can be set and the learners are not overloaded with too many idioms they can neither remember easily nor use naturally.

Techniques and classroom approaches

Idioms and idiomatic language have some characteristics that make certain approaches more worthwhile and productive than others.

Context and co-text
Idioms are, as we saw above, not confined to general informal speech but that is the setting in which they are most frequently used.
It makes sense, therefore, to ensure that we set them in a clear informal social context with language used among people well known to each other and without too many disparities in roles and power.
Idioms occur more naturally in informal story-telling language so that's a good place to situate them. Context will also help learners appreciate issues of style and co-text will often disambiguate meaning successfully.
In particular, idioms are frequently used to sum up a situation and, without the preceding text are almost impossible to understand.
An expression such as:
this is a whole new ball game
is superficially opaque but if it comes at the end of a conversation about possibilities and suggestions, it may be easy to grasp the meaning.
Illustrations
Because idioms are so picturesque, it makes it comparatively easy to link them to illustrations (see the introduction to this guide for examples).
Figurative meaning
Group and pair-work to decipher figurative meaning is helpful because people see different things in language and have different mental metaphors so they can help each other significantly. Obviously, the more transparent the meaning is, the less hard the expression will be to understand and learn.
Retelling stories
Retelling exercises can be a powerful way of consolidating idiomatic language as can dialogue writing and creative story writing.
Translation
Idioms rarely translate exactly between languages but it is surprising how similar concepts are encoded in idiomatic language. For example, the expression it cost an arm and a leg is rendered in Spanish with something like it cost an eye of your face and in German, a hook in the matter means there's a fly in the ointment. All languages exhibit a rich and picturesque range of metaphorical meaning. It can be educational, interesting and fun to compare how meanings are encoded in various languages and that, too, can help retention.
Here are some examples for fun. Can you match the English idioms on the left to their equivalents in other languages on the right? Click on the image when you have an answer.

You can, of course, get your learners to make up quizzes like this and challenge others to find the English equivalent or figure out the meaning.

More teaching ideas can be found in Irujo (1986)

In the section for learners on this site, there are some exercises to do with idioms and binomials. Check the exercise index under vocabulary for more.

There is a very short test on some terms to help you recall some of this.

Related guides
synonymy	for more on how this and related areas work with more on similes and metaphors (fixed and otherwise)
semantics	for a theoretical guide to meaning
teaching lexis	for some practical ideas
PDF list	for all the lists used above of English idioms
the lexical approach	for a guide to an approach to analysing and teaching language which focuses on chunks, holophrases, polywords and more
collocation	for more on this form lexical relationship
classifiers and partitives	for a guide which considers the role of restricted partitives such as pane of and rasher of
empty or delexicalised verbs	for a list in PDF format
multi-word verbs	for the guide to many idiomatic expressions

References:
Barkema, H, 1996, Idiomaticity and terminology: a multi-dimensional descriptive model, Blackwell Publishing Ltd., Studia Linguistica, Volume 50, Issue 2, pp. 125-160
Benor, SB & Levy, R, no date, The Chicken or the Egg? A Probabilistic Analysis of English Binomials, available from http://www.pdfmanuale.com/file/9GW/the-chicken-or-the-egg-a-probabilistic-analysis-of-english.html [accessed January 2015]
lrujo, S, 1986, A piece of cake: learning and teaching idioms, English Language Teaching Journal, 40 (3) pp. 236-242, Oxford: Oxford University Press
Lewis, M, 1993, The Lexical Approach, Hove, England: Language Teaching Publications
Macis, M and Schmitt, N, 2017, The figurative and polysemous nature of collocations and their place in ELT, ELT Journal Volume 71/1 pp50-59, Oxford: Oxford University Press
Moreno, REV, no date, Idioms, Transparency and Pragmatic Inference, available from http://www.phon.ucl.ac.uk/publications/WPL/05papers/vega_moreno.pdf [accessed January 2015]
Schmitt, N and McCarthy, M, 1997, Vocabulary: Description, Acquisition and Pedagogy, Cambridge: Cambridge University Press
Sweet, H, 1889, The practical study of languages, London: Oxford University Press (Reprinted in 1964)
(A list of, it is claimed, over 1500 English idioms is available from https://7esl.com/english-idioms/ but be warned, many are not really idioms and proverbs are included in the lists.)

Spot the idiom	Definitions	Collocation vs. idiom	Lexical chunk vs. idiom	Phrasal verb vs. idiom
Origins	Proverb origins	Fixedness and constraints	Opacity	Duplex expressions
Classification	Opacity and fixedness	One-word idioms	Bi- and tri-nomials	Reduplication
Ricochet words	Frozen similes	Style	Pronunciation	Teaching idioms

Idiomaticity

Definitions

Idiom, collocation, lexical chunks and, phrasal verbs: a blurred area

Collocations

Lexical chunks

Phrasal verbs

Origins

Proverb origins

Fixedness

Flexibility

Constraints

Opacity or non-compositionality

Duplex expressions

Classifying idiomatic language

Fixed idioms

Opaque idioms

Metaphorical or figurative uses

The relationship between fixedness and opacity

One-word idioms

Binomials and trinomials

Frozen or fixed similes

Style and register

Pronunciation

Teaching idiomatic language

Difficulties

Classification and selection for teaching

Lists

Techniques and classroom approaches