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 squadron. 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.
Other air forces, especially those from other Commonwealth countries, have often used similar systems of identification. During the Second World War, when units from other air forces were attached to the RAF; such as the Article XV squadrons (also known as '400 series squadrons'), their squadron codes were often changed, to avoid confusion with RAF units.