Daijo-daijin (Grand minister of state) (太政大臣)
Positioned above the Sadaijin (minister of the left) and the Udaijin (minister of the right), the Daijo-daijin (grand minister of state, also known as Dajo-daijin) was the highest official post in the Daijokan (Grand Council of State) under the Ritsuryo system. The Japanese reading is "omatsurigotonomaetsugimi." Equivalent Chinese posts are Shokoku, Daijosho and Taishi.
With no particular duties, people such as an emperor's teacher (top instructor) were appointed, so it was not always filled, only when an appropriate person was available. For that reason, it was called a "sokketsu post" ("ketsu" means "absent"). Also, it was the only one exempted from danso (indictment for illegal activities) by the danjodai (Board of Censors). A post at the rank of Shoichii (Senior First Rank), Juichii (Junior First Rank).
In the early years of the Ritsuryo system it was treated as a post that could not be filled lightly, and instead an equivalent Chidajokanji was established, to be filled only by members of the imperial family, but later their retainers came to be appointed to Daijo-daijin. Although these conditions lead to a custom of Sadaijin managing Daijokan in place of Daijo-daijin, later on when FUJIWARA no Yoshifusa and FUJIWARA no Mototsune served as both Daijo-daijin and Sessho (regent), there arose some confusion as to the difference in the duties of Daijo-daijin and Sessho, and who would control Daijokan when there were both Daijo-daijin and Sadaijin at the same time. That is when, on June 5, 884, Emperor Koko invited scholars SUGAWARA no Michizane and OKURA no Yoshiyuki to give their opinions of the authority of Daijo-daijin. As a result, it was decided that, as Daijo-daijin was ultimately the Emperor's teacher, a virtuous person who had rendered distinguished service should be appointed, and he would not have political power, and would not get involved in political affairs of Daijokan (however, when also serving as Regent or Kanpaku,chief adviser to the Emperor), he would have political power consequent to that position).
"Nihon Sandai Jitsuroku"(sixth of the six classical Japanese history texts)
As the influence of the Northern House of the Fujiwara clan extended, for the 250 year period from FUJIWARA no Yoshifusa (served 857-872) until MINAMOTO no Masazane (served 1122-1124), the Fujiwara clan maintained a monopoly.
After the end of the Heian period, as a rule only members of Sekke (line of regents and advisers) or the Seiga family (one of the highest court noble families in Japan at that time) were qualified to serve, but in the Edo period it seems that only former Sessho and Kanpaku were appointed to Daijodaijin.
Investiture of Daijo-daijin while living was rare, but as a posthumous honor, it was sometimes given to Sessho and Kanpaku, to maternal grandfathers of the emperor, and to Seii taishogun (literally, "great general who subdues the barbarians") in the Edo period
When the post of Daijo-daijin was given after death for meritorious service performed in life, it was "Zo Daijo-daijin" (the Grand Minister, posthumously conferred). There are also cases of people who were de facto regents to the Shogun, such as Nobunaga ODA, who died without being invested with the title but were given it in a later era (the Taisho period); in fact, immediately after his death he was given the title Juichii Daijo-daijin, but this was changed to Shoichii in the Taisho period.