British military aircraft designations are used to refer to aircraft types and variants operated by the armed forces of the United Kingdom. Since the end of the First World War, aircraft types in British military service have generally been known by a service name (e.g. "Spitfire"), with individual variants recognised by mark numbers often in combination with a letter to indicate the role. This is in contrast to identification systems used in countries such as the United States, where an aircraft type is primarily identified by an alphanumeric designation. The British military aircraft designations (e.g. "Spitfire Mark V" or "Hercules C3") should not be confused with the serial number used to identify individual aircraft (e.g. "XR220"), nor with U.S. aircraft designations (e.g. "C-5", "C-17", "MQ-9") or manufacturer's designations (e.g. "Sikorsky S-58", "Jaguar B", "WS-61", "AW139", "WAH-64"), though Mark numbers were used to indicate aircraft built for other nations e.g. Hawker Hunter Mk 58 was a Hunter F.6 for the Swiss Air Force. No designation system was introduced during World War I that covered more than the products of a single manufacturer. The Admiralty frequently referred to designs by the serial of the first aircraft of that type to be accepted for service. The military designation system In this system, which has been used since the end of World War I, each aircraft designation consists of a name, (sometimes) a role prefix and a mark number. A unified official naming system was introduced in February 1918 by the Ministry of Munitions, the scheme would use classes of names related to the role. Fighter aircraft were to be animals, plants or minerals, bomber aircraft were to have geographical names and "heavy armoured machines" would be personal names from Mythology. The classes were further divided by size of aircraft and land or sea-based, for example a three-seater sea-based fighter would be named after shellfish. Italian towns were to be used for single-seat land-based bombers. Following the formation of the Royal Air Force in April 1918 the Ministry of Munitions introduced a new system as Technical Department Instruction 538. They mainly followed the February 1918 scheme but certain names already used for engines were excluded, for example birds of prey were used by Rolls-Royce Ltd. The names related to zoology, geography and mythology were withdrawn in 1927 and the Air Ministry introduced names with the initial letters relating to role, for example "C" for