The term 'branch of Islam' refers to the two main denominations of the religion: Shia and Sunni. Shia, which has four letters, is the second largest branch of Islam and is based on the teachings of the Prophet Muhammad's son-in-law, Ali. Shiism, which has six letters, is the name given to the beliefs and practices of the Shia branch of Islam. Sunni, which has five letters, is the largest branch of Islam and is based on the teachings of the Prophet Muhammad. Shesm, which has five letters, is a term used to refer to the beliefs and practices of the Sunni branch of Islam.