The exposition in "The Open Window" concerns information about Framton Nuttel's medical condition. We're told that Framton's of a nervous disposition and has been advised by his doctor to spend time in the country to help repair his frayed nerves.
Having been supplied with this information, we now know what kind of a person Framton is and so are better able to understand why he reacts to Vera's shaggy-dog ghost story the way he does. Framton's nervous condition also makes him the ideal mark for Vera's cruel little prank. The reader may not be aware of what's going to happen in the story, but he or she can be certain that the exposition is there for a reason, and that the subsequent climax and resolution will be related to it.
Exposition is the revelation of background information about the characters, settings, or other elements of the text, anything that we might need to know to make the events that take place in the plot make sense. For example, the exposition of this particular story...
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