Sunni Islam (/ˈsuːni, ˈsʊni/) is by far the largest branch of Islam, followed by 85–90% of the world's Muslims. Its name comes from the word Sunnah, referring to the behaviour of Muhammad. The differences between Sunni and Shia Muslims arose from a disagreement over the succession to Muhammad and subsequently acquired broader political significance, as well as theological and juridical dimensions. According to Sunni traditions, Muhammad designated Abu Bakr as his successor (the first caliph). This contrasts with the Shia view, which holds that Muhammad announced his son-in-law and cousin Ali ibn Abi Talib as his successor. Political tensions between Sunnis and Shias continued with varying intensity throughout Islamic history and have been exacerbated in recent times by ethnic conflicts and