As a principle rule of the Imperial succession during the period between Emperor Tenmu and Emperor Shotoku was to succeed the throne from the direct Imperial line. (There was an example of having an Empress in case of the appropriate successor was too young to succeed to the throne.)
天武から称徳に至るまでの皇位継承のルールは（承継すべき者が未成人などの場合には女帝が中継することもあったが）、原則として直系承継であった。