Although there was a custom for the descants of the Imperial Family to make demotion from nobility to subject from the government based on the ritsuryo legal codes, after the Medieval Period, the Imperial Family line (the hereditary Imperial Family) was established, such as Fushiminomiya and Kaninnomiya for the Imperial succession, in order for them to succeed to the throne after many generations.
皇族の子孫は数代経た後に皇籍から離脱するのが律令以来の通例であったが、中世以後、伏見宮や閑院宮の様に皇統維持のために、何代経ても親王位につくことのできる家系（世襲親王家）を創出していったのである。