twenty Hinayana schools ［二十部・小乗二十部］ ( nijū-bu or shōjō-nijū-bu): Also, twenty schools. A term applied in the Mahayana tradition to the schools formed by schisms in the Buddhist Order in India resulting from divergent interpretations of Shakyamuni Buddha’s teachings after his death. According to The Doctrines of the Different Schools, the first schism occurred about one hundred years after Shakyamuni’s death in the wake of controversy over five modifications advanced by a monk named Mahādeva. At that point, the Buddhist community split into two schools: the Sthaviravāda (Pali Theravāda) school, which rejected Mahādeva’s views, and the Mahāsamghika school, which supported them. A Correct Commentary on the Rules of Discipline does not attribute this first schism to Mahādeva’s five modifications, however. Instead, it ascribes the division to a dispute over the “ten unlawful revisions”—ten new interpretations of monastic discipline set forth by the monks of the Vriji tribe in Vaishālī.
According to The Doctrines of the Different Schools, a split within the Mahāsamghika occurred in the second one-hundred-year period after the Buddha’s death. Eventually eight schools derived from the Mahāsamghika, forming nine Mahāsamghika schools: Ekavyāvahārika, Lokottaravāda, Kaukkutika, Bahushrutīya, Prajnāptivādin (also Prajnaptivādin), Chaityavādin, Aparashaila, Uttarashaila, and Mahāsamghika. The Sthaviravāda school divided during the third one-hundred-year period after the Buddha’s death; first the Sarvāstivāda school broke away, while the original Sthaviravāda school called itself the Haimavata school. Thereafter the Sarvāstivāda school gave rise to four more schools: Vātsiputrīya, Mahīshāsaka, Kāshyapīya, and Sautrāntika. The Vātsīputrīya school further divided into the Dharmottara, Bhadrayānīya, Sammatīya, and Shannāgarika schools, and the Mahīshāsaka split to form the Dharmagupta school. Thus there were a total of eleven Sthaviravāda schools. Along with the nine Mahāsamghika schools, they form the twenty Hinayana schools. The schools arising from these schisms are sometimes referred to as the eighteen Hinayana schools, a designation that does not include the two original schools, Sthaviravāda and Mahāsamghika.