Universal Decimal Classification

[1][2][3][4][5] 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.

[11] 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.

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.

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 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[16]).

An advantage of decimal notational systems is that they are infinitely extensible, and when new subdivisions are introduced, they need not disturb the existing allocation of numbers.

[27] 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.

Concepts are organized in two kinds of tables in UDC:[28] The vacant class 4 is the result of a planned schedule expansion.

Indicated in italics below, are special auxiliary numbers that can be used to express attributes (facets) of any specific faith.

Each specific field of technology or industry usually contains more than one special auxiliary table with concepts needed to express operations, processes, materials and products.

Equally, some parts of this class enumerate concepts to a great level of detail e.g. 621.882.212 Hexagon screws with additional shapes.