This rule is said to have been introduced by Yasumori ADACHI, who carried out a reform of the shogunal administration under the banner of the Shogun from the Imperial family and Tokimune Hojo, the regent for the Shogun.
これは皇族将軍と執権北条時宗を擁して幕政改革に乗り出した安達泰盛が採用したといわれている。