The Universal Decimal Classification (UDC) is a bibliographic and library classification representing the systematic arrangement of all branches of human knowledge organized as a coherent system in which knowledge fields are related and inter-linked. The UDC is an analytico-synthetic and faceted classification system featuring detailed vocabulary and syntax that enables powerful content indexing and information retrieval in large collections. Since 1991, the UDC has been owned and managed by the UDC Consortium, a non-profit international association of publishers with headquarters in The Hague (Netherlands).
Unlike other library classification schemes that have started their life as national systems, the UDC was conceived and maintained as an international scheme. Its translation in world languages started at the beginning of the 20th century and has since been published in various printed editions in over 40 languages. UDC Summary, an abridged Web version of the scheme is available in over 50 languages. The classification has been modified and extended over the years to cope with increasing output in all areas of human knowledge, and is still under continuous review to take account of new developments.
Albeit 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 either 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.
The UDC was developed by the Belgian bibliographers Paul Otlet and Henri La Fontaine at the end of the 19th century. In 1895, they created the Universal Bibliographic Repertory (Répertoire Bibliographique Universel) (RBU) which was intended to become a comprehensive classified index to all published information. The idea that the RBU should take the form of a card catalogue came from the young American zoologist Herbert Haviland Field, who was at the time himself setting up a bibliographical agency in Zurich, the Concilium Bibliographicum. A means of arranging the entries would be needed, and Otlet, having heard of the Dewey Decimal Classification, wrote to Melvil Dewey and obtained permission to translate it into French. The idea outgrew the plan of mere translation, and a number of radical innovations were made, adapting the purely enumerative classification (in which all the subjects envisaged are already listed and coded) into one which allows for synthesis (that is, the construction of compound numbers to denote interrelated subjects that could never be exhaustively foreseen); various possible relations between subjects were identified, and symbols assigned to represent them. In its first edition in French "Manuel du Répertoire bibliographique universel" (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.
The Universal Bibliographic Repertory itself has developed into a remarkable information resource. In the period before World War I it grew to more than 11 million records. The catalogue and its content organized by UDC can still be seen in Mundaneum in Mons, Belgium (in 2013 recommended for inclusion in the UNESCO Memory of the World Register).
UDC is used in around 150,000 libraries in 130 countries and in many bibliographical services which require detailed content indexing. In a number of countries it is the main classification system for information exchange and is used in all types of libraries: public, school, academic and special libraries.
UDC code 37.037:796.56, provided in the article "The game method as means of interface of technical-tactical and psychological preparation in sports orienteering" in the Russian journal "Pedagogico-psychological and medico-biological problems of the physical culture and sport" ( ISSN 2070-4798).
UDC code 621.715:621.924:539.3 in the article Residual Stress in Shot-Peened Sheets of AIMg4.5Mn Alloy - in the journal Materials and technology ( ISSN 1580-2949).
The design of UDC lends itself to machine readability, and the system has been used both with early automatic mechanical sorting devices, and modern library OPACs. Since 1993, a standard version of UDC has been maintained and distributed in a database format: UDC Master Reference File (UDC MRF) which is updated and released regularly. The 2011 version of the MRF (released in 2012) contains over 70,000 classes. In the past full printed editions used to have around 220,000 subdivisions.
hierarchically expressive – the longer the notation, the more specific the class: removing the final digit automatically produces a broader class code.
UDC is an analytico-synthetic and faceted classification. It allows an unlimited combination of attributes of a subject and relationships between subjects to be expressed. UDC codes from different tables can be combined to present various aspects of document content and form, e.g. 94(410)"19"(075) History (main subject) of United Kingdom (place) in 20th century (time), a textbook (document form). Or: 37:2 Relationship between Education and Religion. Complex UDC expressions can be accurately parsed into constituent elements.
UDC is also a disciplinary classification covering the entire universe of knowledge. This type of classification can also be described as aspect or perspective, which means that concepts are subsumed and placed under the field in which they are studied. Thus, the same concept can appear in different fields of knowledge. This particular feature is usually implemented in UDC by re-using the same concept in various combinations with the main subject, e.g. a code for language in common auxiliaries of language is used to derive numbers for ethnic grouping, individual languages in linguistics and individual literatures. Or, a code from the auxiliaries of place, e.g. (410) United Kingdom, uniquely representing the concept of United Kingdom can be used to express 911(410) Regional geography of United Kingdom and 94(410) History of United Kingdom.
Common auxiliary tables (including certain auxiliary signs). These tables contain facets of concepts representing, general recurrent characteristics, applicable over a range of subjects throughout the main tables, including notions such as place, language of the text and physical form of the document, which may occur in almost any subject. UDC numbers from these tables, called common auxiliaries are simply added at the end of the number for the subject taken from the main tables. There are over 15,000 of common auxiliaries in UDC.
The main tables or main schedules containing the various disciplines and branches of knowledge, arranged in 9 main classes, numbered from 0 to 9 (with class 4 being vacant). At the beginning of each class there are also series of special auxiliaries, which express aspects that are recurrent within this specific class. Main tables in UDC contain more than 60,000 subdivisions.
The vacant class 4 is the result of a planned schedule expansion. This class was freed by moving linguistics into class 8 in the 1960s to make space for future developments in the rapidly expanding fields of knowledge; primarily natural sciences and technology.
-0...	Common auxiliaries of general characteristics: Properties, Materials, Relations/Processes and Persons. Table 1k.
UDC classes in this outline are taken from the Multilingual Universal Decimal Classification Summary (UDCC Publication No. 088) released by the UDC Consortium under the Creative Commons Attribution Share Alike 3.0 license (first release 2009, subsequent update 2012).
141        Kinds  of viewpoint. Including:  Monism. Dualism. Pluralism. Ontological Materialism. Metaphysical Idealism. Platonism, etc.
The UDC tables for religion are fully faceted. Indicated in italics below, are special auxiliary numbers that can be used to express attributes (facets) of any specific faith. Any special number can be combined with any religion e.g. -5 Worship can be used to express e.g. 26-5 Worship in Judaism, 27-5 Worship in Christianity, 24-5 Worship in Buddhism. The complete special auxiliary tables contain around 2000 subdivisions of various attributes that can be attached to express various aspects of individual faiths to a great level of specificity allowing equal level of detail for every religion.
Universal decimal classification used at the library of the maison Losseau.
This section is currently vacant.
"3"	Conventional time divisions and subdivisions: numbered, named, etc.
"5"	Periodicity. Frequency. Recurrence at specified intervals.
-054	Persons according to ethnic characteristics, nationality, citizenship etc.
^ a b "UDC Fact Sheet". UDC Consortium website. Retrieved 28 October 2018.
^ McIlwaine, I. C. (2009). Universal Decimal Classification (UDC). Encyclopedia of Library and Information Sciences (Third ed.). pp. 5432–5439. doi:10.1081/E-ELIS3-120043532. ISBN 978-0-8493-9712-7.
^ McIlwaine, I. C. (1997). "The Universal Decimal Classification: Some factors concerning its origins, development, and influence". Journal of the American Society for Information Science. 48 (4): 331–339. doi:10.1002/(SICI)1097-4571(199704)48:4<331::AID-ASI6>3.0.CO;2-X.
^ Slavic, A. (November 2004). "UDC Translations: a 2004 Survey Report and Bibliography". Extensions & Corrections to the UDC. 26 (2004): 58–80. hdl:10150/106363.
^ Slavic, A.; Cordeiro, M. I.; Riesthuis, G. (June 2008). "Maintenance of the Universal Decimal Classification: overview of the past and preparations for the future". International Cataloguing and Bibliographic Control. 37 (2): 23–29. hdl:10150/105220.
^ Slavic, A. (2004). "UDC implementation: from library shelves to a structured indexing language". International Cataloguing and Bibliographic Control. 33.3 (2004): 60–65. hdl:10150/105685.
^ UNESCO Memory of the World Register, "Universal Bibliographic Repertory - UNESCO Memory of the World Register"
^ Slavic, A. (2006). "Use of the Universal Decimal Classification: a worldwide survey". Journal of Documentation. 64 (2): 211–228. hdl:10150/105579.
^ Slavic, A. (December 2006). "The level of exploitation of Universal Decimal Classification in library OPACs: a pilot study". Vjesnik Bibliotekara Hrvatske. 49 (3–4): 155–182. hdl:10150/105346.
^ Slavic, A. (2006). "UDC in subject gateways: experiment or opportunity?". Knowledge Organization. 33 (2): 67–85. hdl:10150/105276.