A call number is the address of each book or journal within the library. It is like a street address within a city. Call numbers also place books and journals on the same subject next to each other on the shelf. There are several systems of call numbers; for example, the Dewey Decimal Classification System and the Library of Congress Classification System are the two primary systems. The Virginia Tech Libraries use the Library of Congress Classification System, except for US federal government documents. These are shelved according to the Superintendent of Documents (SUDOC) document numbering system.