The Shiites are a group of Muslims who follow the teachings of the Prophet Muhammad's cousin and son-in-law, Ali. They make up one of the two major branches of Islam, the other being the Sunnis. There are several synonyms for the word Shiite, with the most common being Shia. Other terms include Shiism and Shia Islam. The word Twelver is also sometimes used to describe Shiite Muslims who believe that there were 12 imams after the Prophet Muhammad, with Ali being the first. Additionally, the term Shi'a is sometimes used to refer collectively to all followers of Ali, regardless of their specific beliefs or practices.