war and the first to have a defining specification. To reduce the vulnerability of early German military aircraft to Allied types equipped with machine guns, "C" types were armed with a rearward firing machine gun operated by the observer and (later) a forward firing synchronised machine gun for the pilot. An engine of more than 150 hp was also specified (later "C" types typically had over 200 hp). A number of future German fighter aces obtained their first victories in a "C" type aircraft CL Lightweight "C" class aircraft (designation introduced early 1917). Later "C" types became progressively larger – the "CL" specification was intended to provide smaller aircraft, nimble enough to be used as escorts for the heavier "Cs": effectively two-seat fighters. In practice, the "CL" types were mainly used for close support. Engine power of a "CL" was limited to less than – and total loaded weight to under . In other respects "CL" types were similar to "C"s – in fact serial and type numbers generally fell in the same sequence. CN "C" class aircraft modified to carry a heavier bomb load for use at night. Designation replaced by "N". D Single-seat armed aircraft, specifically intended for use by the new jagdstaffeln (fighter) squadrons (designation introduced in 1916). Until late in 1918, when the system was rationalised, "D" implied an armed doppeldecker (biplane). By the end of the war all single-seat fighters were designated as "D" types, as distinctions based on wing layout were abandoned. DJ Armoured "D" class aircraft. The only type in this class was the prototype AEG DJ.I and the designation may not have been official. Dr Single-seat armed triplane, (Dreidecker ). Designation introduced in late 1917 and abandoned in late 1918. The first two Fokker Dr.I service test aircraft were in fact called "F.I". By the end of the war all new single-seat fighters became "D" types, regardless of wing layout. E Armed monoplane (designation introduced in 1915 – abandoned in late 1918), from Eindecker. This was initially simply the monoplane version of the "C" class armed biplane, having the same relationship to the "C" class as the "A" had to the "B", and several early "E" types were two-seaters. In practice, due largely to the success of the single seat Fokker "E" types, which were single-seat fighters, the "E" class came to mean a single-seat fighter monoplane (i.e. the monoplane equivalent of