of a book or periodical, a list, nearly always alphabetical, of the topics treated. This list is usually at the back of a book, and the table of contents is in the front. The index seeks to direct the reader to all names and subjects on which the book has information. The subject, with the number of the page on which related information is to be found, is called the entry. In an index to a periodical the entries are less specific, referring usually to an article as a whole rather than to every subject touched upon in each article. Indexing requires experience and skill, since it is necessary not only to grasp the meaning of the author but to phrase that meaning clearly and in such a way as to place it alphabetically where the reader is likely to look first. Books written to give information are of little value unless properly indexed. Indexes to books were made long before the invention of printing. In the 16th cent. the term index began to be commonly applied to such a list; until the 17th cent. the index was rarely alphabetical. Diderot's famous Encyclopédie (1751–1772) had an alphabetical index. In 1848 in the United States a general index to the most widely circulated periodicals of the time was issued by William Frederick Poole. Poole's Index, later compiled cooperatively, continued until 1907, when it was superseded by the Readers' Guide to Periodical Literature. There are special indexes in various fields of knowledge, e.g., law, medicine, art, education, engineering, industrial arts, agriculture. Newspaper indexes include those to the London Times (from 1906) and the New York Times (from 1851). Indexes are increasingly being compiled by computer, and published as on-line databases and in CD-ROM format. The H. W. Wilson and R. R. Bowker companies are noted for special annual indexes, particularly the Readers' Guide to Periodical Literature, the Cumulative Book Index, and Ulrich's International Periodicals Directory. Indexes to illustrations, to artifacts, to formulas, and to various collections of materials are common. Some are alphabetical; others may be by number, color, or some other scheme. The catalog of the books in a library is sometimes known as an index.

See M. D. Anderson, Book Indexing (1971); R. L. Collison, Indexes and Indexing (4th ed. 1972); J. Rowley, Abstracting and Indexing (2d ed. 1988); D. B. and A. D. Cleveland, Introduction to Indexing and Abstracting (2d ed. 1990).

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