The Sunni-Shia split is rooted in the question of who should succeed the Prophet Muhammad in leading Muslims after his death in 632.

Shias say the prophet's cousin and son-in-law, Ali, was his rightful successor but he was cheated when authority went to those the Sunnis call the four "Rightfully Guided Caliphs" — Abu Bakr, Omar, Othman and, finally, Ali.

Sunnis are the majority across the Islamic world. In the Middle East, Shias have strong majorities in Iran, Iraq and Bahrain, with significant communities in Lebanon, Yemen, Syria, Saudi Arabia, Kuwait and other parts of the Gulf.

Iraq, which is in danger of exploding in sectarian bloodshed owing to targeted killings of Shias by the al-Qaeda offshoot ISIS, is 60 to 65 per cent Shia, and 32 to 37 per cent Sunni, according to the CIA's World Fact Book.

Both consider the Qur'an the word of God. But theology and religious practice distinguish between the two sects.

Some differences are minor: Shias pray with their hands by their sides, Sunnis with their hands crossed at their chest or stomach.

Others are significant. Shias, for example, believe Ali and a string of his descendants, the Imams, had not only rightful political authority after Muhammad but also held a special religious wisdom. Most Shias believe there were 12 Imams — many of them "martyred" by Sunnis — and the 12th vanished, to one day return and restore justice.

Accused of elevating Ali

Sunnis accuse the Shias of elevating Ali to the level of Muhammad himself — incorrectly, since Shias agree that Muhammad was the last of the prophets, a central tenet of Islam.

Sunni and Shia population estimates:

The number of Shia Muslims worldwide in 2010 was estimated to be between 162 million and 211 million. The number of Sunni Muslims was projected to be around 1.4 billion. Sunnis are expected to make up 87-90 per cent of the world’s Muslims in 20 years, roughly the same percentage as today.

Source: Pew Research Center, 2010 Religion and Public Life Project

The bitter disputes of early Islam still resonate. Even secular-minded Shia parents would never name their child after the resented Abu Bakr, Omar or Othman — or Aisha, a wife of Muhammad who helped raise a revolt against Ali during his Caliphate.

When outgoing Iranian president Mahmoud Ahmadinejad visited Egypt last year, the sheik of Al-Azhar, the bastion of Sunni theology, told him sharply that if the sects are to get along, Shias must stop "insulting" the "companions of the prophet."

Still, only the most hardcore translate those differences into hatred. But events in Iraq over the past decade show how politics can cause long-simmering tensions to spike.

The U.S.-led invasion that toppled Saddam Hussein's Sunni-dominated regime in 2003 propelled long-oppressed majority Shias to power. Sunnis feared the repression would flip onto them. The result was vicious sectarian fighting that lasted until 2008: Sunni insurgents pulled Shia pilgrims from buses and gunned them down; Shia militiamen kidnapped Sunnis, dumping their tortured bodies on the streets and in rivers.

The bloodshed ebbed after the so-called U.S. surge, a revolt by moderate Sunnis against al-Qaeda in Iraq and a Shia militia cease-fire. But violence has again increased as minority Sunnis feel disenfranchised by the Shia-led government and Sunni extremists have been emboldened by the successes of Islamic militants in the civil war next door in Syria.

According to the Pew Research Institute, the number of Shia Muslims worldwide in 2010 was estimated to be between 162 million and 211 million. The number of Sunni Muslims was projected to be around 1.4 billion.

With files from CBC News