The Universal Decimal Classification is a system of library classification developed by the Belgian bibliographers Paul Otlet and Henri la Fontaine at the end of the 19th century. It is based on the Dewey Decimal Classification, but uses auxiliary signs to indicate various special aspects of a subject and relationships between subjects. It thus contains a significant faceted or analytico-synthetic element, and is used especially in specialist libraries. UDC has been modified and extended over many years to cope with the increasing output in all disciplines of human knowledge, and is still under continuous review to take account of new developments.

The documents classified by UDC may be in any form. They will often be literature, i.e. written documents, but may also be in other media such as films, video and sound recordings, illustrations, maps, and realia such as museum pieces.

UDC classifications use Arabic numeral system and are based on the decimal system. Every number is thought of as a decimal fraction with the initial decimal point omitted, which determines filing order. For ease of reading, a UDC identifier is usually punctuated after every third digit. Thus, after 61 "Medical sciences" come the subdivisions 611 to 619; under 611 "Anatomy" come its subdivisions 611.1 to 611.9; under 611.1 come all of its subdivisions before 611.2 occurs, and so on; after 619 comes 620. An advantage of this system is that it is infinitely extensible, and when new subdivisions are introduced, they need not disturb the existing allocation of numbers.

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