Melvil Dewey created the Dewey Decimal Classification system when he was 21 years old to organize library books into 10 main categories represented by 3-digit call numbers. The system divides all knowledge into 10 classes like philosophy, religion, social sciences, and geography that are printed on the spine of books so they can be easily shelved in order. Each class has subclasses to further categorize topics within that field of knowledge.