Generally, aircraft in British military service were known by names assigned by their manufacturer, or (for various imported types) bestowed upon them by the first military service to bring them into service. There was a period (in the 1920s) when names followed function, beginning with 'F' for fighters, 'N' for naval, 'B' for bomber, and so on. Often the Navy would simply preface the RAF name with the word "Sea" (for example Sea Hurricane or Sea Heron).
From about 1910, it was decided that all aircraft for British Army use would be designed at the Royal Aircraft Factory, Farnborough, although they might be built elsewhere. These did have reasonably consistent designations. The Admiralty chose to have private industry design and build its aircraft. The Army eventually relented, and also bought industry-designed aircraft.
From 1920 to 1949, every type had an associated Air Ministry Specification number. Some of these never produced a prototype, let alone an aircraft in service. Others were drawn up around a private venture design, or an imported model.
Variants of each operational type are normally indicated by letters to indicate the current function of that aircraft and then a number indicating the sequence in which that variant achieved operational status. No number is reused with a different functional prefix. For example the first Lockheed Hercules in RAF service was known as the C.1. A later version with a lengthened fuselage received the designation C.3 because a single example adapted for weather monitoring purposes had already taken the designation W.2. Aircraft with a long service life may find that their function changes from time to time and a change in the designation letters and sometimes the following digit will reflect such new roles.
These functional prefixes are: