Rank succession - Ikaikeisho

…First of all, during the Nara period, the aim was for the succession of the Emperor to the legitimate line (from the legitimate son to the legitimate grandson), and for that purpose, with several interim empresses in between, the legitimate descendants of Emperor Tenmu continued to pass through until the late Nara period (Imperial succession). For the class of government officials, the Succession Law clearly stipulated that the succession should be through the legitimate line, but this was solely for the purpose of the succession of ranks (innocent ranks), and did not mean the succession of an actual family with Chinese-style succession of rituals and family headship. It is assumed that the actual group that existed behind the succession of ranks was the clan. …

*Some of the terminology explanations that refer to "rank succession" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…まず奈良時代には，天皇の地位は嫡系継承(嫡子から嫡孫へ)が目指され，そのために間に何人かの中継ぎの女帝を挟みつつ，天武天皇の嫡系の子孫が奈良後期まで続く(皇位継承)。官人層については，継嗣令に明確に嫡系継承が規定されているが，それはもっぱら位階継承(蔭位（おんい）)のためであって，中国流の祭祀相続･家長権を伴った実体としての〈家〉の継承を意味するものではない。位階継承の背後に存在した現実の集団としては〈氏〉が想定される。…

※「位階継承」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報