The Catalogue was replaced in 2012 by a new service called Discovery. An archive catalogue is not like a library catalogue. The difference must be understood before undertaking any search. In libraries books are organized by subject classifications, i.e. history, biographies, but archives are arranged by collections or in the case of the The National Archives by the government department or branch of the English legal system which created or collected them. These are indicated by letter codes (e.g. WO - War Office, C - Chancery, or WARD - Court of Wards and Livery). Within each departmental letter code the records are divided into separate series, indicated by series numbers. The series numbers are usually assigned sequentially and have no significance other than assisting in finding records. A department letter code plus series number is called a class number (e.g. WARD 4). Class or series numbers are further divided into bundles or pieces. Within a piece, records are often numbered by page, sheet, folio, quire or some other reference number.