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. 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 Poole's Index, later compiled cooperatively, continued until 1907, when it was superseded by the Readers' Guide to Periodical Literature.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.