The Universal Decimal Classification (UDC) is a bibliographic and library classification developed by the Belgian bibliographers Paul Otlet and Henri La Fontaine at the end of the 19th century. UDC provides a systematic arrangement of all branches of human knowledge organized as a coherent system in which knowledge fields are related and inter-linked.
Originally based on the Dewey decimal classification, the UDC was developed as a new analytico-synthetic classification system with a significantly larger vocabulary and syntax that enables very detailed content indexing and information retrieval in large collections. In its first edition in 1905, the UDC already included many features that were revolutionary in the context of knowledge classifications: tables of generally applicable (aspect-free) concepts – called common auxiliary tables; a series of special auxiliary tables with specific but re-usable attributes in a particular field of knowledge; an expressive notational system with connecting symbols and syntax rules to enable coordination of subjects and the creation of a documentation language proper. Although originally designed as an indexing and retrieval system, due to its logical structure and scalability, UDC has become one of the most widely used knowledge organization systems in libraries, where it is used for shelf arrangement, content indexing or both. UDC codes can describe any type of document or object to any desired level of detail. These can include textual documents and other media such as films, video and sound recordings, illustrations, maps as well as realia such as museum objects.