One issue was it should be someone from the Imperial family line, another issue was it should be someone who has a maternal relationship to the former Emperor. (Emperor Konin was the husband of Emperor Shotoku's half younger sister)
一つは必ず皇胤である事、もう一つは先代の天皇との婚姻関係を有する（光仁は称徳の異母妹の夫）ことであった。