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Universal Decimal Classification

(Contributed by Antoine Isaac and Aida Slavic, UDC Consortium)

The Universal Decimal Classification (UDC, is a classification scheme whose aim is to cope with all fields of knowledge in a structured way, and is used for document and information organization in libraries, documentation and information services and bibliographic databases.

Top level of UDC consists of nine main classes corresponding to the main disciplines or knowledge areas (, which are further subdivided into subclasses. Each main class is represented with decimal numerical notation separated by point after every three digits e.g. the class of Differential Galois theory is assigned a notation 512.628.25.

5               Mathematics and natural sciences
51              Mathematics
512             Algebra
512.6           Special branches of algebra
512.62          Fields. Polynomials
512.628         Fields with additional structures      
512.628.2       Differential algebra
512.628.25      Differential Galois theory

Additionally to the hierarchical links resulting from subdivision, the UDC system allows relational organization of vocabulary into facets within the single discipline - separating the main subject (main facet hierarchy) from concepts that can be used for further specification (attributes) within the field (processes, operations, tools, materials) - these class numbers are called special auxiliaries.

For example, the class 2, Religion has the following set of main subdivisions:

21      Prehistoric and primitive religions
22      Religions of the Far East
23      Religions of the Indian subcontinent
24      Buddhism
25      Religions of antiquity. Minor cults and religions
26      Judaism
27      Christianity
28      Islam
29      Modern spiritual movements

But class 2 also points to a series of special auxiliaries:

-1      Theory and philosophy of religion. Nature of religion. Phenomenon of religion
-2      Evidence of religion
        -23     Sacred books. Scriptures. Religious texts
        -24     Specific texts. Named texts and books 
-3      Persons in religion
-4      Religious activities. Religious practice
-5      Worship broadly. Cult. Rites and ceremonies
-6      Processes in religion
-7      Religious organization and administration
-8      Religions characterised by various properties
-9      History of the faith, religion, denomination or church

These can that can be applied to 2 as well as its specializations, as in the following examples:

24      Budhism
24-24           The Pali Canon. Tipitaka. Three 'baskets'
24-244                  Vinaya pitaka. Basket of discipline

27      Christianity 
27-242          The Old Testament
27-242.2                The Pentateuch

Apart from auxiliaries whose application is limited to specific main classes, UDC contains additional nine facets of generally applicable concepts or common auxiliary numbers (languages, form, place, ethnic grouping, time, properties, materials, relations, persons). Classes in these tables are assigned notations that consist of decimal numbers enclosed in various symbols/signs called facet indicators - which makes them distinct from the main numbers ( These "common auxiliary numbers" that are universally applicable can be: used on its own, combined among themselves or with main numbers:

main numbers
        314     Demography
        656.1   Road transport  

common auxiliaries
FORM    (084.3) Cartographic images. Maps. Plans. Charts
PLACE   (100)   Universal as to space. International. All countries
TIME    "17"    18th century

combination of common auxiliairies among themselves and with main numbers
"17"(100)(084.3)        18th century - maps - world 
314"17"(100)(084.3)     demography - 18th - maps - world 
656.1"17"(100)(084.3)   road transport - 18th - maps - world 

Any two main UDC numbers may be used to express complex subject e.g. 656.1 Road transport and 504.7 Global warming of atmosphere may be combined using : (colon) e.g. 656.1:504.7 to express the subject "relationship between road transport and global warming". In addition common auxiliaries of relations, for instance, may be used to denote a type of binary relationship between two subjects. Taking for example subjects 004, Computer science and 37, Education together with relationship auxiliaries

  -042.2   Comparison phase
  -042.3   Influence phase

one can synthesize the following subjects:

37:004 unspecified relationship between computer science and education
37-042.2:004 comparison of 'computer science' and 'education'
37-042.3:004 influence of 'computer science' on 'education'

Finally, the Master Reference File of the standard UDC edition (in 2006: 54 678 Main numbers and 12.929 Common auxiliaries) provides class entries with scope notes, application notes, editorial notes, cross-references and examples of combination. Since 1993 UDC MRF is maintained and distributed to users as an export file in English only but various printed and electronic editions of the UDC exist in 39 languages (see:;