This is a result of new common laws being formed in various areas of ritsuryo law, due to a variety of historical changes such as the emergence of new political formats like sekkan seiji (politics of regents and advisers) and insei (rule by a retired Emperor), decline of ancient farmland allotment and accepting system and development of manors, and dismantling of the class system under ritsuryo law.
摂関政治や院政などの新しい政治形態の出現、班田制の衰退と荘園の発展、律令法的身分秩序の解体などにみられる各種の歴史上の変化によって、律令法に基づく新しい慣習法が律令法の各分野で形成されてきた結果である。