The Shia, or the Shiites, represent the second largest denomination of Islam. Adherents of Shia Islam are called Shias or the Shi'a as a collective or Shi'i individually. Shi'a is the short form of the historic phrase Shīʻatu ʻAlī meaning "followers", "faction" or "party" of Muhammad's son-in-law and cousin Ali, whom the Shia believe to be Muhammad's successor in the Caliphate. Twelver Shia is the largest branch of Shia Islam, and the term Shia Muslim is often taken to refer to Twelvers by default. Shia Muslims constitute 10-13% of the world's Muslim population and 11-14% of the Middle East's entire population. Shi'i Islam is based on the Quran and the message of the Islamic prophet Muhammad attested in hadith recorded by the Shia, and certain books deemed sacred to the Shia. Shia consider Ali to have been divinely appointed as the successor to Muhammad, and as the first Imam.