Most units of the Royal Air Force (RAF) are identified by a two character alphabetical or alpha- numeric combination squadron code. Usually, that code is painted on the aircraft belonging to that unit. The squadron code is usually presented along with an individual letter or character to form a call sign for the particular aircraft. Location of the call sign combination has usually been on the rear fuselage next to the RAF roundel. In instances when an unusually large numbers of aircraft comprise the squadron, multiple squadron codes have been used.