Melvil Dewey created the Dewey Decimal Classification (DDC) system when he was 21 years old to bring order to the organization of books in libraries. The DDC system divides all knowledge into 10 main categories, with each category represented by 3 numbers. These 10 categories include Generalities, Philosophy, Religion, Social Science, Language, Science and Math, Technology, Arts, Literature, and Geography and History. The DDC call number can be found on the spine of each library book to easily identify and locate the book when shelved.