The card catalog in a library contains cards that provide information about books in the library's collection. The cards are organized alphabetically and contain identifying information like call numbers to help users locate books on the shelf. There are three types of cards: author cards list books alphabetically by the author's last name, title cards list books alphabetically by title, and subject cards list books under broad topics that are further subdivided into more specific subjects. This system was created to help patrons find books regardless of whether they know the author, title, or subject.