Ritsuryo law remained the basic national law throughout the Nara and Heian periods, but important changes occurred around the time when "Engishiki" (a book of laws and regulations compiled during the Engi era) was established and issued in the tenth century.
律令法は、奈良・平安両時代を通じて国家の基本法であることに変りはなかったが、10世紀の『延喜式』の制定公布の時代前後を境として、重要な変化がみられた。