Dewey Decimal Classification, also called Dewey Decimal System, system for organizing the contents of a library based on the division of all knowledge into 10 groups, with each group assigned 100 numbers. The 10 main groups are: 000–099, general works; 100–199, philosophy and psychology; 200–299, religion; 300–399, social sciences; 400–499, language; 500–599, natural sciences and mathematics; 600–699, technology; 700–799, the arts; 800–899, literature and rhetoric; and 900–999, history, biography, and geography. These 10 main groups are in turn subdivided again and again to provide more specific subject groups. Within each main group the principal subseries are divided by 10; e.g.,