Chugu is one of the names given to wives of Japanese emperors.
The meaning of the term 'chugu' is a 'residence of an empress'. The empress who lives there is also called 'chugu'. It was originally a Chinese word and had the same meaning in China. Since Empress Dowager and Grand Empress Dowager were in the same rank of empresses, 'chugu' was used as the name of the residence of an empress and the empress herself. The Taiho Ritsuryo (Taiho Code) stipulated that Chugushiki (Office of Imperial Wives) should be established in the Ministry of Central Affairs as an organization of government officials to serve three empresses all together; Empress, Empress Dowager, and Grand Empress Dowager.
Period in which chugu was Kotaifujin (title for previous retired emperors' wife)
After the establishment of Taiho Ritsuryo (Taiho Code), female emperors were enthroned successively and empresses were not appointed, thus Chugushiki did not work in fact. The only male emperor during the period, Emperor Monmu, did not put up an empress because the highest-ranking wife (actual legal wife) was FUJIWARA no Miyako Fujin (consort of the emperor).
When Emperor Shomu was enthroned in 724, his real mother Miyako was called Kotaifujin, but Chugushiki was established at this time to serve Kotaifujin Miyako. She was treated in the same way as an empress, and it was considered to be a special treatment taken by the emperor to increase the authority of her mother who was not an empress. This measure was possible because there was no one in the position of the empress at the time. Miyako was often called chugu because she was served by Chugushiki.
The treatment of Chugushiki assigned to Miyako became a problem when Empress Komyo was put up as an empress in 729. According to the Ryo regulation, Chugushiki should be assigned to Empress Asukabehime. However, the emperor made a decision to establish a new Kogogushiki (the Queen-consort's Household Agency) as Ryoge no kan (a post outside the original Ritsuryo code created by Imperial edicts) and let Chugushiki continue to serve Kotaifujin Miyako. Miyako was given a title of Grand Empress Dowager when her grandchild Empress Koken was enthroned, and continued to be served by Chugushiki until her death in 754. Kotaigo gushiki (Imperial Household Agency assigned to the household of Queen Mother) was established for Asukabehime who became Empress Dowager when Emperor Shomu abdicated the throne and served her consistently until she died in 760.
These events became precedents, and Kogogushiki, Kotaigo gushiki, Taikotaigogushiki (Imperial Household Agency assigned to the household of Grand Empress Dowager), and Chugushiki were established to serve Empress, Empress Dowager, Grand Empress Dowager, and Kotaifujin respectively. Taikotaigogushiki was established for the first time for FUJIWARA no Junshi who became Grand Empress Dowager in 864 according to historical documents.
After Chugushiki became an organization of government officials dedicated to Kotaifujin, chugu was also used exclusively as the name of Kotaifujin. Seven Kotaifujin after Miyako, from TAIMA no Yamashiro, the real mother of Emperor Junnin, to FUJIWARA no Onshi, the foster mother, appeared and were served by Chugushiki. Four of them became Empress Dowagers later, and were served by Kotaigogushiki which was established for each of them.
Period in which chugu was empress
After FUJIWARA no Onshi died in 907, no one held the position of Kotaifujin. Six generations of emperors did not put up empresses for about 100 years after the investiture of the Empress to Imperial Princess Masako, empress of Emperor Junna, but Emperor Daigo put up nyogo (a high-ranking lady in the court [a consort of an emperor]) FUJIWARA no Onshi as empress to revive the empress after a long time. Chugushiki, not Kogogushiki, was established to serve Onshi. This was the first case in which Chugushiki was assigned to an empress. Then, Chugu became the name of an empress for the first time. Onshi became Empress Dowager when Daigo died and their son Emperor Suzaku was enthroned, and became Grand Empress Dowager when Emperor Suzaku demised the throne to his maternal half-brother Emperor Murakami, but she was always served by Chugushiki and called chugu throughout the time. This was the first case in which Chugushiki was operated according to the Ryo regulation. However, it became the last case at the same time. Empress of Emperor Murakami, FUJIWARA no Anshi, and Empress of Emperor Reizei, Imperial Princess Shoshi, were also served by Chugushiki and called chugu.
When Emperor Enyu put up nyogo FUJIWARA no Koshi as empress in 973, he changed Empress Masako to Empress Dowager and revived Kotaigogushoku to serve her. Chugushiki was established for Koshi and it was thereafter settled as an organization of government officials dedicated to empresses. Chugu became to exclusively be used as the name of empresses. FUJIWARA no Junshi who became an empress instead of Koshi who died young was also served by Chugushiki and called chugu. Kogogushiki was not established during this period.
Period in which chugu and Kisai no miya (empress) coexisted
FUJIWARA no Teishi, a daughter of FUJIWARA no Michitaka, entered into court for Emperor Ichijo and became nyogo in 990. Sessho (regent) Kaneie, the father of Michitaka, died and Michitaka became Sessho and wished to put up Teishi as empress to reinforce his political power, but the post of the empress was not vacant because Junshi stayed as empress even after the abdication of Emperor Enyu, and FUJIWARA no Senshi, the real mother of Ichijo, was Empress Dowager and Masako was Grand Empress Dowager. As a matter of course, no one could be dismissed from the position of the empress unless she committed a crime or misconduct. Since the empress was originally a 'lawful wife of an emperor' (an interpretation of the articles of the Ryo in "Ryo-no-gige" [commentary on the Ryo]), Michitaka forcibly put up Teishi who was the lawful wife of the reigning emperor, as empress, while Junshi, the wife of 'Daijo tenno' (the Retired Emperor) Enyu, stayed as empress. It transpired that there were two empresses at the same time. Chugushiki which had been assigned to Junshi was assigned to Teishi, and Kogogushiki was revived and assigned to Junshi. Since the distinction between the names of the two empresses became a problem, Junshi was called Kisai no miya (empress) and Teishi was called 'chugu' according to the offices of government officials assigned to them.
When FUJIWARA no Michinaga became a person in paramount authority after the death of Michitaka and the downfall of his legitimate son FUJIWARA no Korechika, Teishi became a powerless empress due to the downfall of her family home, even if she was the mother of Imperial Prince Atsuyasu, the first prince of Ichijo. On the other hand, Michinaga wished to have his daughter Jotomonin enter into court for Ichijo, make her nyogo and then empress. Ichijo loved Teishi who moaned about the downfall of Korechika, retired from the Imperial Palace and entered into priesthood so much that he had her enter into court again regardless of criticism of people, and although he was reluctant to the investiture of the Empress to Akiko, he was persuaded by those around him to accept the intention of Michinaga in the end. Junshi was shifted to the vacant post of Empress Dowager and Kotaigogushoku was established for her in 1000, and Kotaigogushoku was established instead of Chugushiki and assigned to Teishi, and Akiko was put up as empress to which Chugushiki was assigned. Teishi was called Kisai no miya (empress), and Akiko, chugu. This became a precedent that one emperor could have two empresses.
Two empresses were allowed until the end of the Kamakura period, one was called Kisai no miya (empress) and the other, chugu. However, there were not always two empresses. It became an institution that an honorary title of empress was given to a woman who was not a 'lawful wife of an emperor'.
Both Kisai no miya and chugu were essentially empresses between whom there was no difference or hierarchical relationship. However, hanretsu (order of seats in court) was in the order from Kisai no miya to chugu. There are no clear rules about usage of Kisai no miya and chugu. However, there is a rough trend that follows.
If an emperor put up one of his wives as empress, she was called chugu.
If the emperor put up another wife as the second empress, the first one became Kisai no miya and the second one was called chugu.
An empress that did not meet the conditions of 'lawful wife of an emperor', such as young emperor's Junbo (a woman who was given the status equivalent to the emperor's birth mother) and a wife of retired emperor, was called Kisai no miya.
This is an approximate trend with a lot of exceptions. Empress of Emperor Goreizei, FUJIWARA no Kanshi, was the second empress of Goreizei, and when she became an empress, Kanshi was called Kisai no miya and the first empress Imperial Princess Shoshi stayed as chugu. After Shoshi became Kisai no miya, Kanshi was transferred from Kisai no miya to chugu. Imperial Princess Teishi who became an empress as junbo of Emperor Horikawa was called chugu though she was not a 'lawful wife of an emperor' and remained single her entire life.
The empresses were not installed after the period of the Northern and Southern Courts, and the names of Kisai no miya and chugu were not used. A daughter of Hidetada TOKUGAWA, Tofukumon-in who became the lawful wife of Emperor Gomizunoo was put up as empress from nyogo in 1624, and the empress was revived after the lapse of about 300 years and she was called chugu. There were three other empresses in the Edo period, who were all 'lawful wives of emperors' and called chugu. Two empresses were no longer put up at the same time and the name of Kisai no miya was not used.
Empress Dowager Shoken who became empress in 1868 was called chugu first, but renamed to Kisai no miya the next year. The name of 'chugu' was not included in the Former Imperial House Law established in 1889, here, chugu was abolished and Haruko became the last chugu. At the same time, the number of empress to be authorized at a time was fixed to one.