This unwritten rule was originally made to avoid any trouble that might occur in Imperial succession, Emperor Gomizunoo used this to stop the Tokugawa lineage from the Imperial lineage and intended not to have any Tokugawa blood in the Imperial family, thus he made his daughter, Empress Meisho to succeed the throne. (However Okiko's younger sister, Imperial Princess Akiko and Imperial Princess Yoshiko married into the Konoe family and the Nijo family, two of the five families whose members were eligible for the positions of Sessho and Kanpaku.)
この決まりは元来、皇位継承の際の混乱を避けることが主要な意図であるが、後水尾天皇はこの不文律を利用し、天皇家から徳川の血を絶やし、後世までその累が及ばぬようにするという意図などをもって、娘の明正天皇を即位させたといわれている（ただし興子の同母妹の昭子・賀子両内親王はそれぞれ五摂家の近衛家・二条家に降嫁している）。