After WWII, medieval Buddhism studies in Japan were led by the theory that old Buddhism, Nanto rokushu (the six sects of Buddhism which flourished in ancient Nara) and Heian nishu (the two sects of Buddhism that flourished in the Heian period: the Tendai and Shingon sects), co-existed separately and exclusively with equal power.
戦後の中世仏教史では、旧仏教である南都六宗と平安二宗（天台宗、真言宗）は対等な存在として対立的または排他的な関係のもとに併立しているとの見方が大勢を占めていた。