Most units of the Royal Air Force are identified by alphabetical (or similar) characters, known as a "squadron code", that is painted on all aircraft belonging to that unit. When individual units are assigned unusually large numbers of aircraft, 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.
Historically, the codes have usually been two letters of the alphabet, painted on the rear fuselage next to the RAF roundel. These formed a suffix or prefix to the call sign of each aircraft (on the other side of the roundel) which was usually a single letter (e. g. "G for George"). In general, when an aircraft is lost or withdrawn from use, its call sign has been applied to its replacement or another aircraft.
Data from: Flying Units of the RAF
* = RAAF/RCAF unit that was not under RAF operational control.
Transport Command RAF applied the following three letter call-signs to its aircraft from 1944. Individual aircraft were given letter suffixes, making the code a four letter sequence.
Data from: Flying Units of the RAF
From 1945 RAF Transport Command pre-fixed the three letter call-signs with M. With individual aircraft allocated letter suffixes, the call-signs became five letter sequences. Data from:
Royal Air Force
Army Air Corps
Fleet Air Arm
Others