The Imperial Japanese Navy Air Service (大日本帝國海軍航空隊, Dai-Nippon Teikoku Kaigun Koukuu-tai, IJNAS) used several different aircraft designation systems simultaneously.
The Mitsubishi Zero was so-called because entered service in 1940 which was the Japanese year 2600, thus it was designated Type 0 Carrier Fighter.
[4] Sometimes two aircraft were ordered from different manufacturers to the same specification at the same time, generally as insurance against the primary design not working out.
Special cases include aircraft that employed non-conventional (i.e. non-propeller-driven) propulsion scheme like rocket-powered interceptor Shūsui (poetic term meaning "Sharp Sword") and aircraft used for non-conventional deployments such as Special Attacker Seiran ("Mountain Haze"; deployed from submarines to strike targets behind the frontline and expected to be ditched upon returning to motherships).
The Imperial Japanese Army Air Service (often called the Imperial Japanese Army Air Force (大日本帝國陸軍航空隊、大日本帝國陸軍航空部隊, Dainippon Teikoku Rikugun Kōkūtai, Dainippon Teikoku Rikugun Kōkūbutai) (IJAAS) used a straightforward system based on year of service and type, nearly identical to the Navy's long type and model number system.
Assigned letters included 甲 (Ko) for Nieuport, 乙 (Otsu) for Salmson, 丙 (Hei) for SPAD, 丁 (Tei) for Farman, 戊 (Bo) for Caudron, and 己 (Ki) for Hanriot.
The first part of the designation was a two-digit type number based on the Japanese year in which the aircraft entered service.
Small-scale modifications (such as armament) are indicated with a Japanese Kanji ordinal from the Heavenly stems:- ko (甲), otsu (乙), hei (丙), tei (丁), bo (戊), ki (己), which equate to:- a (first), b (second), c (third), d (fourth), e (fifth), but are NOT direct translations.
This is a sortable table giving all the various designations and names of Japanese Military aircraft from circa 1925 to 1945.