Dewey Decimal Classification

The Dewey Decimal Classification (DDC; , ), colloquially known as the Dewey Decimal System, is a proprietary library classification system which organizes books based on discipline and subject. It was first published in the United States by Melvil Dewey in 1876. Originally described in a 44-page pamphlet, it has been expanded to multiple volumes and revised through 23 major editions, the latest printed in 2011. It is also available in an abridged version suitable for smaller libraries. OCLC, a non-profit cooperative that serves libraries, currently maintains the system and licenses online access to WebDewey, a continuously updated version for catalogers.

The decimal number classification made use of the concepts of relative location and relative index. This contrasted to the library practice in which books were given permanent shelf locations that were related to their order of acquisition rather than topic. The classification's notation makes use of three-digit numbers for main classes, with fractional decimals allowing expansion for further detail. A library assigns a classification number that unambiguously locates a particular volume in a position relative to other books in the library, on the basis of its subject. The number makes it possible to find any book and to return it to its proper place on the library shelves. As of 2009 the classification system is used in 200,000 libraries in at least 135 countries.