Alids are the descendants of Ali, the cousin and son-in-law of the Islamic prophet Muhammad, the fourth Sunni Rashidun caliph and the first Imam in Shia Islam. The two principal branches of the Alid lineage are the Hasanids and Husaynids, named after Hasan and Husayn, the sons of Ali and Fatima, the daughter of Muhammad. The direct line of Alids, beginning with Ali himself, constitutes the Twelve Imams of Twelver Shi'ism, the largest branch of Shia Islam. As the family of Muhammad, they are revered by all Muslims.