rigid airships from (the Zeppelin Company) and as well as various types of airship from other firms. Aircraft designation system During the First World War, German aircraft officially adopted for military service were allocated a designation that included (1) the name of the manufacturer, (2) a function or "class" letter, and (3) a Roman numeral. The three-part designation was needed for a unique designation to simplify logistics support of the many types of aircraft in operation – especially as squadrons more often than not were equipped with several different types. The designation system evolved during the war. Initially, all military aircraft were classed as "A" (monoplanes) or "B" (biplanes). The new "C" class of armed (two seat) biplane began to replace the "B" class aircraft as reconnaissance machines in 1915, the Bs continuing to be built, but as trainers. The "E" class of armed monoplane was also introduced in 1915 – the other classes were added later as new aircraft types were introduced. For most of the war 'D' was only used for biplane fighters, 'E' for monoplane fighters and 'Dr' for triplane fighters, however by the end of the war the 'D' designation was used for all single-seat fighters, including monoplanes (and, in theory at least, triplanes). A – Unarmed reconnaissance monoplane aircraft (for example the Rumpler Taube and Fokker M.5) B – Unarmed two-seat biplane, with the observer seated in front of the pilot. C – Armed two-seat biplane, with the observer (usually) seated to the rear of the pilot. CL – Light two-seater (primarily from Halberstadt and Hannover), initially intended as escort fighters – by 1917–18, mainly used for ground attack. D – – single-seat, armed biplane but later any fighter – for instance the Fokker E.V monoplane was renamed the D.VIII. Dr – – triplane fighter (twin service test Fokker triplanes initially "F") E – – armed monoplane – initially included monoplane two-seaters. New monoplane types at the end of the war designated as "D" (single seat) or "CL" (two seat). G – – Large twin-engined types, mainly bombers (initially "K") GL – Lighter, faster twin-engined bombers, intended for use by day. J – – Fuel tanks, pilot, and (usually) the engine protected by armour plate, reducing vulnerability to ground fire. Used for low-level work, especially ground attack. N – "C" type aircraft adapted for night bombing – apart from night flying equipment they were fitted