"Kugyo" is a term referring to court noble posts responsible for national politics as the top-ranking officers of Daijokan (Grand Council of State) based on provisions of the Ritsuryo system (a system of centralized government based on the ritsuryo code), namely, the top officials (collectively called giseikan), Daijo-daijin (Grand minister of state), Sadaijin (minister of the left), Udaijin (minister of the right), Dainagon (chief councilor of state), Chunagon (vice-councilor of state), and Sangi (imperial advisor) (or Jusanmi (Junior Third Rank) or higher). They began being called kugyo during the Heian period.
"Kugyo bunin," a list the personnel assigned to these posts, was created annually, and was used until Daijokan was abolished during the Meiji period. The Ritsuryo System was put into place after the sankokyukei (administrative posts in the Qin and Han eras in China) in the Chinese rules governing administrative organizations, and the high officials were called kugyo. Courtiers at the Imperial Court were called kugyo hyakkan (100 kugyo positions) as a whole. When military rule was established and the pretension of government services went unchallenged, bukekani (official court titles for samurai), hyakkanna (court rank-like names used by warriors), and azumahyakkan (court rank-like names used by warriors in the Kanto region), which were identities imitating those for government services were established (the Tokugawa Shogun family and Tokugawa gosanke (three privileged branches of Tokugawa family) were promoted to Jusanmi or higher to qualify for kugyo despite being buke families).
As the title of honor for kizoku (noble), ason was added after the family name and ko was added after the first name for Daijodaijin, Sadaijin, Udaijin, Naidaijin (inner minister), and for kizoku or Sangi official of the third rank or higher, ason was added after the family name and kyo was added after the first name to honor them. For those who were at the fourth rank and had not reached Sangi, ason was added after their full names. Kugyo was also referred to as hoshino kurai (position of the stars), kandachime, unjobito (people above the clouds). Third rank or higher was called ki and fifth rank or higher was called tsuki.
When sekke (line of regents and advisers) were established among court nobles starting at the end of the Heian period, and the social status and family status became fixed, the so called genpeitokitsu, or the Minamoto clan, Taira clan, Fujiwara clan, and Tachibana clan, became the only four family names allowed to become kugyo (Later, the Sugawara clan, Abe clan, and Kamo clan were allowed, but limited to those in the lineage of eldest son).
In the Sengoku Period (Period of Warring States), some local daimyo (Japanese territorial lords) in the Sengoku Period and their vassals announced themselves using these family names to be granted court ranks authorized by the Imperial Court. The court ranks of these samurai families were called bukekani.
Nobunaga ODA was originally from the Inbe clan, but is said to have changed his family name to Taira due to the genpeikotairon (Minamoto and Taira clans rotation theory).
Hideyoshi TOYOTOMI first referred to himself as Taira, but received the Fujiwara family name by becoming the adopted child of the Konoe family, one of Gosekke (five top Fujiwara families whose members were eligible for the positions of Sessho and Kanpaku), and became kanpaku (chief advisor to the Emperor). Later, he was granted the Toyotomi family name, which is the fifth family name, by Emperor Ogimachi.