The Decimal Classification, devised by Melvil Dewey, is the oldest of the classification schemes still in common use, and by far the most widely used. It is the scheme that most non-librarians are familiar with, and indeed few of them will be aware of any other library classification.

Although it is the oldest scheme, I have chosen to place it after the Library of Congress Classification, since the latter is a somewhat simpler scheme to apply and doesn't show the complexity of operation that DDC now does. DDC is an enumerative scheme with analytico-synthetic features and as such sits quite neatly between LCC and UDC.

DDC is used all over the world and in all types of libraries, although most British readers will think of it as the ‘public library’ classification, since there is only one British public library which doesn't use it. It's also the predominant classification in school libraries. As it becomes more detailed and with more facilities for number building, it has increased in popularity as a scheme for academic libraries. Despite the fact that its publishers claim it to be used by 20% of special libraries in the US, it's probably not sufficiently specific for many special libraries or those with a great deal of technical material.

DDC is quite exceptionally well supported institutionally, and this is undoubtedly one of the reasons for its tremendous success. DDC classmarks appear on all Library of Congress catalogue records, and its editorial office is located there, which enables close monitoring of trends in documentation and the emergence of new topics. It has for several decades been the classification used in the British National Bibliography, and it was adopted by the British Library for the organization of the Reading Room collections at the new library when it opened.

OCLC, the Online Computer Library Center, a not-for-profit organization, publishes the classification, and maintains WorldCat, a two billion item database of records, which include DDC data, from 10,000 libraries across the globe. OCLC also undertakes research and has a number of interesting projects in the area of vocabulary management and automatic indexing, some of which are concerned with developing new roles and applications for the classification.

Formal maintenance and revision of the classification is undertaken by the editor-in-chief and assistant editors under the guidance of the Editorial Policy Committee.