This is a partial list of the British Air Ministry (AM) specifications for aircraft. A specification stemmed from an Operational Requirement, abbreviated "OR", describing what the aircraft would be used for. This in turn led to the specification itself, e.g. a two-engined fighter with four machine guns. So for example, OR.40 for a heavy bomber led to Specification B.12/36. Aircraft manufacturers would be invited to present design proposals to the ministry, following which prototypes of one or more of the proposals might be ordered for evaluation. On very rare occasions, a manufacturer would design and build an aircraft using their own money as a "private venture" (PV). This would then be offered to the ministry for evaluation. The ministry may well release a specification based on the private venture aircraft if the plane aroused interest from the RAF or the ministry due to its performance or some other combination of features. The system of producing aircraft to a specification ran from 1920 to 1949 during which the Air Ministry was replaced by first the Ministry of Aircraft Production (MAP) in 1940 and then the Ministry of Supply (MoS) in 1946. The system was applied to commercial aircraft as well, two being the de Havilland Comet and Vickers Viscount. During the period, over 800 specifications were issued. Specification designations Each specification name usually followed a pattern. A leading letter was usually present to identify the aircraft purpose. The codes used included B for "heavy bomber", e.g., B.12/36, P for "medium bomber", e.g., P.13/36, F for "fighter", e.g., F.10/35, and A for "army co-operation", e.g., A.39/34. The second part was a number identifying it in sequence and then after the slash, the year it was formulated, so in the example given above, B.12/36 signifies a specification for a heavy bomber, the twelfth specification of all types issued in 1936. Specifications were not always issued in sequence. Admiralty specifications were identified by the letter N (Naval), e.g., N.21/45, and experimental specifications identified by the letter E (Experimental), e.g., E.28/39, with training aircraft signified by the letter T (Training), e.g., T.23/31, and unpowered aircraft, signified by the letter X, e.g., X.26/40. The letter G (General) signified a general-purpose aircraft, e.g., G.9/45, with an M (Multi-role) being applied to aircraft intended for more than one specific purpose, e.g., M.15/35. The letter C (Cargo) was applied to military transport aircraft, e.g., C.1/42, with the letter O