Universal Decimal Classification System - Quick reference to the numerical subject headings used in the Universal Decimal Classification System, from 00 (Generalities) to 94 (History).
Universal Decimal Classification Consortium - Organisation maintaining and promoting Universal Decimal Classification, a system of cataloging and indexing books and knowledge developed by Paul Otlet at the end of the 19th century.
Wikipedia: Universal Decimal Classification - Overview of Universal Decimal Classification, a method of large-scale library cataloging and indexing based on the Dewey Decimal system.
