At a time when only a Tenmu-descendent prince could ascend the throne, Emperor Kanmu was born as the first prince of Tenchi-descendent Emperor Konin (Prince Shirakabe at that time) but Rittaishi (investiture of the Crown Prince) was not conducted, although it was normally done at the time of birth if the person's order of succession was high.
天武系でなければ即位すら出来なかった時代に天智系の光仁天皇（当時は白壁王）の第一皇子として生まれ、立太子は行われなかった（通常、継承順位が高ければ生まれると同時に行われた）。