After the library selects resources for its collection, it organizes them for easy retrieval. A classification system is the method used by libraries to organize materials. When you receive an e-mail message or magazine to your home, the information gets to the right place because you have a unique, one-of-a-kind address. The same principle holds for library materials. Each item that enters the building must be issued a unique "address" (call number) so it can be efficiently and quickly accessed.