Empresses Consort
In ancient Japan most of empresses consort were princesses, except for Iwa-no hime (empress consort of Nimmyō). After Kōmyō Kōgō (empress consort of Shōmu), daughters of Fujiwara clan or other clans could become empresses consort. Originally Chūgū (中宮) meant the palace for Kōgō, Kōtaigō (皇太后) (Empress Mother/Empress Dowager), or Tai-Kōtaigō (太皇太后) (Grand Empress Mother/Grand Empress Dowager). Until mid-Heian Period, Emperor had only one empress consort, and empress consort was also called Chūgū. Since Emperor Ichijō, because some Emperors had two empresses Consort, one of them was called Kōgō and the another one was called Chūgū. After maiden Princess Yasuko became Kōgō as the mother-in-law (准母) of Emperor Horikawa, maiden princesses also became Kōgō.
Read more about this topic: Japanese Empresses