Fiction books are classified by the author's last name, while fiction videos are classified by title. Nonfiction materials are classified according to the Dewey Decimal Classification system (DDC21). The nonfiction call number consists of the classification, followed by a second line of letters and/or numbers which is called a cutter. If the item is shelved in a special location, the call number is preceded by a collection code, such as M for mystery fiction, REF for reference, GEN for genealogy, and pb  for paperbacks