Koi (the Position of Empress) (后位)
Koi (the Position of Empress) indicates Grand Empress Dowager, Empress Dowager or Empress that was the position given to a legitimate wife of the present or previous retired Emperor. These three positions are generally named "Sangu"(the three Princesses) or "Sango" (the three Empresses).
According to Ritsuryo (the Legal Codes), Koi which was based on the blood or marriage relationship to Emperor, was established as following. The title Grand Empress Dowager was given to Emperor's grandmother who had had the title Koi of Empress Dowager and of Empress; the title Empress Dowager was given to Emperor's mother who had had the title Koi of Empress; and the title Empress was given to Emperor's legitimate wife. These titles corresponded to the ranks higher than "Hii" and "Fujini"; "Hii" was the title for "Hi" (the selected Princess from the Imperial family), "Kotaihi" (Emperor's mother who had been also "Hii") and "Taigotaihi" (Emperor's grandmother who had been also "Hii"); "Fujini" was the title for Fujin (the selected woman from the high-ranked family), "Kotaifujin" (Emperor's mother who had been also "Fujini") and "Taikotaifujin" (Emperor's grandmother who had been also "Fujini").
However, once the Heian period started, the blood or marriage relationship with Emperor was ignored due to the complicated succession to the Imperial Throne caused by the abdication of several Emperors, which constantly led to elevate the lower position of a woman to "Koi" in the changing of Emperor and also to appoint who was "Koi" on the grounds of political background. It was not until the Imperial House Act was established in the Meiji period that these situations were resolved.