A library classification is a system of coding and organizing library materials according to their subject. A classification scheme consists of tables of subject headings and classification schedules used to assign a class number to each item being classified, based on that item's subject.
Until the 19th century, most libraries had closed stacks, so classifications were used to organize subject catalogues. Once libraries opened their stacks to the public, they started to shelve library material itself according to a library classification to simplify subject browsing.
An alphabetical list of subject headings is produced, and numbers are assigned to each heading in alphabetical order.
Divides subjects into mutually exclusive facets. Multiple classifications may be assigned to an object, allowing searching and browsing of related information through several classes. Elements may include subject, geographical, temporal and form of an item.
Divides subject hierarchically, from the most general to the most specific.