A call number is a set of letters and numbers that is used by a library to identify a book and to indicate the exact subject(s) covered by its contents. It serves as a sort of "social security" number for books and other library materials helping to distinguish each item as unique and distinct within the library collection, giving it a specific spot on the shelves. Like most public libraries, the MATC campus libraries use the Dewey Decimal Classification (DDC) system to create Dewey Decimal call numbers for its library materials.