Universal Decimal Classification

The Universal Decimal Classification is one of the most widely used knowledge organization systems in the world, distinguished from other library classification schemes by its analytico-synthetic and faceted design. Unlike purely enumerative systems that list all subjects explicitly, UDC provides a notation with connecting symbols and syntax rules that enable classifiers to construct compound expressions representing complex and interrelated subjects that could never be exhaustively foreseen.

Background

The UDC originated in 1895 when Belgian bibliographers Paul Otlet and Henri La Fontaine created the Universal Bibliographic Repertory (Repertoire Bibliographique Universel), intended as a comprehensive classified index to all published information. The idea for a card catalogue format came from the American zoologist Herbert Haviland Field. After obtaining permission from Melvil Dewey to translate the Dewey Decimal Classification into French, Otlet and La Fontaine introduced radical innovations that transformed the purely enumerative DDC into a system capable of synthesis -- constructing compound numbers to represent interrelated subjects.

The first edition, Manuel du Repertoire bibliographique universel (1905), already included revolutionary features: common auxiliary tables for aspect-free concepts, special auxiliary tables for field-specific attributes, and an expressive notational system with connecting symbols and syntax rules. The Universal Bibliographic Repertory grew to more than eleven million records before World War I. The catalogue, organized by UDC, can still be seen in the Mundaneum in Mons, Belgium, and was accepted onto the UNESCO Memory of the World international register in 2013.

Purpose and Scope

UDC covers all fields of human knowledge and can describe any type of document or object to any desired level of detail -- textual documents, films, video and sound recordings, illustrations, maps, and museum objects. It is used in approximately 150,000 libraries in 130 countries and in many bibliographical services requiring detailed content indexing, including national bibliographies of around 30 countries.

Large databases indexed by UDC include NEBIS (2.6 million records, Switzerland), COBIB.SI (3.5 million records, Slovenia), the Hungarian National Union Catalogue MOKKA (2.9 million records), and the VINITI RAS database (28 million records, Russia).

Main Classes

The classification uses nine main classes (class 4 has been vacant since the 1960s, when linguistics was moved to class 8 to make room for expanding fields in the natural sciences and technology):

Notation and Syntax

UDC notation uses Arabic numerals arranged decimally, punctuated after every third digit for readability. Two key properties make the notation distinctive:

• Hierarchically expressive -- longer notation indicates more specific classes; removing the final digit produces a broader class
• Syntactically expressive -- connecting symbols interrupt digit sequences to indicate compound expressions

The connecting symbols include:

Common Auxiliary Tables

Common auxiliaries represent aspect-free concepts applicable across all main classes, introduced by distinctive facet indicators. There are over 15,000 common auxiliaries in UDC:

• = Language (e.g., =111 English)
• (0...) Document form (e.g., (075) Textbook)
• (1/9) Place (e.g., (410) United Kingdom)
• (=...) Human ancestry, ethnic grouping, nationality
• "..." Time (e.g., "19" 20th century)
• -02 Properties
• -03 Materials
• -04 Relations, processes and operations
• -05 Persons and personal characteristics

These combine freely: 94(410)"19"(075) = History of United Kingdom in the 20th century, a textbook. Or: 2(075)=111 = Religion textbooks in English.

Special Auxiliary Tables

At the beginning of each main class, series of special auxiliaries express aspects recurrent within that specific class. The religion tables (class 2) exemplify this well: around 2,000 special auxiliary subdivisions express attributes of faiths (theory, evidences, persons, activities, worship, processes, organization, properties, history) that can be combined with any religion -- so -5 Worship produces 26-5 Worship in Judaism, 27-5 Worship in Christianity, or 24-5 Worship in Buddhism, all at equal levels of detail.

Scale and Editions

The Master Reference File (MRF), the standard machine-readable version maintained since 1993, contained over 70,000 classes in its 2011 release. Historical full printed editions contained approximately 220,000 subdivisions. The main tables hold more than 60,000 subdivisions. The UDC Summary, an abridged web version of approximately 2,600 top-level classes, is available in over 50 languages under a Creative Commons Attribution Share Alike 3.0 license.

Governance and Maintenance

Since 1991, the UDC has been owned and managed by the UDC Consortium, a non-profit international association of publishers headquartered in The Hague, Netherlands. The classification is under continuous review, with revisions published through the Extensions and Corrections series and major revision projects addressing individual classes. Unlike other library classifications that began as national systems, UDC was conceived and maintained as an international scheme from its inception, with translation into other languages beginning at the start of the twentieth century.

Related Standards

• Dewey Decimal Classification (DDC) -- the parent classification from which UDC was derived, sharing the basic decimal structure but differing substantially in its auxiliary apparatus and combinatory capabilities

Further Reading

• About UDC
• UDC Summary (free online)
• UDC Linked Data
• Extensions and Corrections