Library classification schedules provide a means for library materials to be organized in subject order. This facilitates physical browsing as well as computer access. Since the time of the Library of Alexandria during the Hellenic period, some logical order has been imposed upon library collections. It is only in the last century and a half that a number of modern notations for library classification have been developed. Such classifications contain letters, numbers, or a combination of the two, to indicate subjects and subtopics; and additional marks to aid alphabetical shelving within the subject. Perhaps the best known is Melvil Dewey's Decimal Classification, first published in 1876. It is composed of ten large classes:

General works (000) Philosophy (100) Religion (200) Social sciences (300) Philology (400) Pure science (500) Useful arts/applied science (600) Fine arts (700) Literature (800) History/biography (900).

A modified version of Dewey's Decimal Classification, called the Universal Decimal Classification, was prepared by the Institut International de Bibliographie (Brussels) in 1899-1905. It is used widely in Europe, and has a more minute identification of subjects than the Dewey classification, as well as incorporating symbols to express the interrelationship of subjects.

Alphanumeric classification schemes were developed about the same time as those that were solely numeric. Charles Ammi Cutter, librarian of the Boston Athenaeum, published his Expansive Classification in 1891. The best features of this scheme were adapted to the Library of Congress Classification. The Library of Congress collections, founded in 1800, were destroyed in 1814 with the burning of the Capitol Building by the British in the War of 1812. In 1815 Congress obtained the library of Thomas Jefferson, and adopted his classification, which was an adaptation of Francis Bacon's scheme for the classification of knowledge. In 1897, the Library of Congress realized that it needed a new classification scheme, and set about to create a pragmatic organization of books that was never expected to be used by other libraries. The outline of the main classes is as follows:

A Polygraphy B-BJ Philosophy BL-BX Religion C Auxiliary sciences of history D History (except North, South and Central Americas) E-F History: America (North, South and Central) G Geography, anthropology H Social science J Political science K Law L Education M Music N Fine Arts P-PA Philology, Linguistics, Classical literature PB-PH Modern European languages PG Russian literature PJ-PM Languages and literature of Asia, Africa, Oceania, America, Mixed languages, Artificial language PN, PR, PS, PZ Literature (General), English and American literature, Fiction and juvenile literature in English PQ French, Italian, Spanish and Portuguese literature PT German, Dutch and Scandinavian literatures Q Science R Medicine S Agriculture T Technology U Military science V Naval science Z Bibliography, librarianship

The Library of Congress Classification has been adopted by most university and college libraries in North America, as well as many large public libraries.