system of arrangement adopted by a library to enable patrons to find its materials quickly and easily. While cataloging provides information on the physical and topical nature of the book (or other item), classification, through assignment of a call number (consisting of class designation and author representation), locates the item in its library setting and, ideally, in the realm of knowledge. Arranging similar things in some order according to some principle unites and controls information from various sources.

Classification can be distinguished by type: (1) natural, or fundamental—e.g., books by subject, (2) accidental—e.g., chronological or geographic, and (3) artificial—e.g., by alphabet, linguistic base, form, size, or numerical order. Degree of classification (e.g., close, with the most minute subdivisions, or broad, with omission of detailed subdivisions) may also characterize a system. Several systems of classification have been developed to provide the type of access and control that a particular library and its clientele need. Generally, each system consists of a scheme that arranges knowledge in terms of stated principles into classes, then divisions and subdivisions.

Current predominating systems include the Dewey Decimal Classification, the Library of Congress Classification, the Bliss Classification, and the Colon Classification (qq.v.); many special and research libraries devise their own unique systems.