Universal Decimal Classification

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 nineteenth century. It was based on the Dewey Decimal Classification, but is much more powerful. It uses auxillary signs to indicate various special aspects of a subject and relationships between subjects. It thus contains a significant faceted or analytico-synthetic element.

It is used especially in specialist libraries.