Grand Empress Dowager (太皇太后)
Grand empress dowager is an honorific title for the legal wife (Empress) of the late Emperor but one or for the grandmother of the present Emperor. So, it once meant the honorific title or the rank of former Empresses in Japan. Now, a Grand empress dowager is called with the title of honor 'Heika' (her Majesty) but in ancient times, 'Gu' (palace) was attached after their names as the title of honor.
Summary
In Japan, this title came into use after the introduction of the Ritsuryo system (a system of centralized government based on the ritsuryo code). Ryo (administrative code) assigned Chugu-shiki (Office of the Consort's Household) to manage the Grand empress dowager's affairs, quoting Ryo no gige (commentary on the Ryo) as its legal basis which says that 'the Grand empress dowager is Chugu (Empress) accordingly', but, in fact, Taikotaigogu-shiki (Office of the grand empress dowager) was established to serve Grand empress dowager exclusively. In the late Heian period when Emperors changed frequently in a short period of time, there were cases that a former imperial wife (Empress) was given the title of Empress dowager when a new Empress came into the palace, and she would be elevated to the title of Grand empress dowager when another new Empress came.
Even after the Ritsuryo codes were abolished in Meiji period, the title of Grand empress dowager continued and a Grand empress dowager is specified as a member of the Imperial family by the Imperial House Act. The title of honor for Grand empress dowager is 'Heika'. Rescript on Imperial Funeral Ceremonial (No. 11 of the Imperial family's Act in 1926) states that Minister of the Imperial Households shall announce the death of a grand empress dowager and decide her posthumous title and that the emperor shall host her funeral.
After the World War II, the former Imperial House Act was replaced by the new Imperial House Act, which was enforced in the same year as the Constitution of Japan. Grand empress dowager is specified as a member of the Imperial family (Article 2) and she may become a regent (Article 17). Her title of honor shall be Heika (Article 23) and she shall be buried in a mausoleum (Article 27).
FUJIWARA no Masaruko, the Empress of Emperor Konoe (she later became the Empress of Emperor Nijo) in the late Heian period, was the last Japanese grand empress dowager up to the present date.