General: Like all navies the Imperial Japanese Navy (IJN) used signal flags for all kinds of purposes and like all navies they used a mix of international flags and pennants, and special IJN flags and pennants. They divided their signal flags into two broad categories, which were, in turn further divided into five sub sets.
The first category was known as Letter or Number flags (Goki) and consisted of two sub sets:
Special Navy Number Flags (Sujiki): The Number Flag set consisted of ten navy special flags unique to the IJN that represented the numbers 0-9.
International Letter Writing Flags (Mojiki): The International Roman alphabet (Romaji) flag set was the same set in use today. It consists(ed) of the 26 letters in the Roman alphabet.
The Letter or Number Flags (Goki) were used to make signals in coded letter/number groups, and set out numbers for course, bearing, distance, speed and the like as commonly used in naval signaling. Most navies have an equivalent to the Navy Number Flags.
The second category was known as Special flags (Kubetsuki) and consisted of three sub sets:
Special Naval maneuver flags (Undokubetsuki): It consisted of ten triangular pennants (which included the three international repeaters) and generally performs the same functions as the USN/RN etc. flags and pennants: corpen, turn, formation, etc.
Ships, Fleets, and Divisions flags (Kanshomeikubetsuki). It consisted of fourteen pennants. Ten of these are also the international numeral pennants, thus serving dual functions. The balance are unique to the IJN. This set corresponds roughly to the division, squadron etc flags in use in the USN and RN.
Special classification flags (Shingokubetsuki). It consists of nine pennants. One of those pennants is also the international answering pennant. The balance are unique to the IJN. The set corresponds roughly to USN and RN pennants for interrogatory, prep, etc.
This paper will treat the flags by category and sub set as follows:
Navy Number Flags; the Arabic number, Japanese pronunciation of the Arabic number, some examples of use and a hira gana reading.
The Roman letter (Romaji), IJN phonetic reading, examples of use, some explanatory notes and kana reading.
Special Flags (Kubetsuki), the Kanji reading (abbreviated and long version), kana readings of certain flags, doctrinal statements (Yoho) on how the flags are to be used in signals, and examples of signals with reference to doctrine.