Designed as an ever-evolving encyclopaedia, the Directory comprises thousands of files. Started in 1907 by Otlet and La Fontaine, the Universal Documentation Directory (UDD) is intended to be a living encyclopaedia, i.e. one that is constantly updated. It is made up of thematic files (classified using the Universal Decimal Classification), and biographical and geographical files: “made up of parts of documents, extracts from publications, without transformation or alteration; small printed documents that can not be placed in the Library are also inserted”. These may be press cuttings, pamphlets, brochures, cut out excerpts from books, passages copied by hand and even archive documents. The aim is to cover all areas of knowledge.

En 1914, the UDD already contained a million documents archived in 10,000 files. Although less systematically, it was added to until the 1980s by Otlet and La Fontaine’s successors.