For those unfamiliar with the coding system, two letters were separated from a third by the roundel. For example, CG - A, where "CG" is the code and "A" is the aircraft identifying letter and usually the radio call sign.

The two-letter squadron code was introduced after the Munich Crisis of 1938. However, many squadrons used a different code following the outbreak of hostilities. This second code did not always extend to non-operational aircraft held on charge by a squadron and so it was common for two separate codes to be displayed. Additionally, the same code combination was in use by more than one squadron at the same time.