A source which is more or less completely dependent on primary sources for its existence is a secondary source. Information in secondary sources is organised and arranged according to a definite plan. Indexing and abstracting periodicals are one of the examples of secondary sources. Indexing periodical, indexes the contents of periodicals or some other type of publications usually on regular basis, whereas abstracting periodical along with the contents also gives the abstract. The abstract can be indicative or informative. Indexing and abstracting periodicals may be either general in nature or on a specific theme. In any abstracting and indexing periodical you will notice that articles of the same topic have all been put together, and of related topics close by. They are no more scattered. Even if the articles are indifferent languages you will find the abstracts in the same language. In a way it is overcoming the language barrier. As the bibliographical details of the primary sources are usually given in the secondary sources, these sources act as a key to primary sources. In a review article, also a secondary source, the entire information on a particular topic for a certain period is first collected, then digested and finally a report is written wherein the entire matter is organised coherently. Here the entire phenomenon of scattering is done away with and the whole matter gets well-organised. Generally most reference books also draw materials from primary sources.