Melvil Dewey created the Dewey Decimal Classification System in 1873 when he was 21 years old. The system divides all knowledge into 10 main categories, with each category represented by 3 numbers. The categories are Generalities, Philosophy, Religion, Social Science, Language, Science & Math, Technology, Arts, Literature, and Geography & History. The Dewey Decimal Classification number can be found on the spine of each library book and helps to organize books on the shelves by subject.