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The 'Idflieg designation system' was used to generate German military aircraft designations prior to the end of World War I.
Each designation consisted of one of the following letters, followed by a roman numeral. For example, the first "D"-class aircraft built by Fokker was designated the Fokker D.I, the second, the Fokker D.II.

★ 'A' - single-seat unarmed aircraft, powered by an engine of less than 150 hp (112 kW)

★ 'B' - two-seat unarmed biplane, powered by an engine of less than 150 hp

★ 'C' - two-seat unarmed biplane, powered by an engine of more than 150 hp

★
★ 'CL' - lightweight "C" class aircraft, nimble enough to be used as fighter aircraft when armed.

★ 'D' - single-seat armed aircraft, powered by an engine of more than 150 hp

★ 'E' - single-seat armed aircraft, powered by an engine of less than 150 hp

★ 'F' - single-seat armed triplane, powered by an engine of less than 150 hp (early use only)

★ 'G' - armed biplane bomber aircraft with two or three engines (''grosse'' - "large"). These aircraft were originally designated 'K' (see below).

★ 'J' - two-seat ground attack aircraft

★ 'K' - armed biplane bomber aircraft with two or three engines (''Kampfflugzeug'' - "battle aircraft"). Later changed to 'G' (see above).

★ 'N' - two-seat night (''Nacht'') bomber

★ 'R' - armed biplane bomber aircraft with four engines (''Riesenflugzeug'' - "giant aircraft").

★ 'W' - seaplanes and flying boats of all kinds. (''Wasser'' - "water")
It is a popularly-held misconception that the 'D' and 'E' designations referred exclusively to ''Doppeldecker'' ("biplane") and ''Einedecker'' ("monoplane") respectively, but not all D-class aircraft were biplanes and not all E-class were monoplanes. This misconception has been furthered by the widespread use of 'Dr' (''Dreidecker'' - "triplane") as an alternative to 'F' (the famous Fokker Dr.I originally more formally known as the 'F.I'), and by the fact that the most famous D-class and E-class aircraft did indeed fit the wing arrangements that these letters are purported to stand for.
The system fell into disuse with the end of German military aviation after the Armistice, and aircraft of the newly reborn Luftwaffe would be designated according to the RLM aircraft designation system.