What Is the Difference Between Sunni and Shia Islam?

When Muhammad died in 632 he left no universally accepted instruction about who should lead the community. One group held that the leader, or caliph, should be chosen by the community's senior figures, and they chose Abu Bakr, the Prophet's close companion and father-in-law. They came to be called Sunni, from a word meaning the tradition or established practice.

Another group held that Muhammad had designated his cousin and son-in-law Ali, and that leadership belonged in the Prophet's family. They became the Shia, from shi'at Ali, the party of Ali. Ali did eventually become the fourth caliph in 656, but he was assassinated in 661 and the caliphate passed to the Umayyad dynasty. The wound became permanent in 680 at Karbala, in present-day Iraq, where Ali's son Husayn and a small band of followers were killed by the caliph's army. Shia Muslims still mourn that day, Ashura, every year.

The deepest theological difference follows from the succession. Shia Islam holds that Ali and a line of his descendants were imams: divinely guided leaders with authority to interpret the faith. The largest Shia group, the Twelvers, recognise twelve imams and believe the last went into hiding in the ninth century and will return. Ismailis follow a different line and today recognise the Aga Khan as a living imam. Zaydis, mostly in Yemen, recognise a shorter line and are closer to Sunni practice.

Sunni Islam has no equivalent figure. Religious authority rests with scholars trained in one of four schools of law, and no single person or institution speaks for all Sunnis. Shia Islam, by contrast, developed a ranked clergy, with senior scholars given the title ayatollah, and in Iran since 1979 that clergy has held political power directly. Twelver Shia also gather at the shrines of the imams, in Najaf, Karbala and Mashhad, in a way that has no exact Sunni parallel.

The two branches share the five pillars: the declaration of faith, prayer, charity, fasting in Ramadan and pilgrimage to Mecca. The differences a visitor would notice are small. Shia Muslims often combine the five daily prayers into three sessions and pray with their arms at their sides, resting the forehead on a small clay tablet; Sunnis usually pray five separate times with arms folded. The call to prayer differs by a phrase. Shia law allows temporary marriage, which Sunni law forbids.

The two traditions also rely on different collections of hadith, the reported sayings of the Prophet, because each trusts different chains of early transmitters. That produces differences in inheritance law, ritual detail and the religious calendar, but the shared ground is far larger than the differences, and for most of Islamic history the two communities lived, traded and intermarried side by side.

Sunnis make up roughly 85 to 90 percent of the world's Muslims. Shia Muslims are a majority in Iran, Iraq, Bahrain and Azerbaijan, form large communities in Lebanon, Yemen, Pakistan, India, Afghanistan and Saudi Arabia's eastern province, and are a small minority almost everywhere else. Iran became a Shia state only in the sixteenth century, when the Safavid dynasty made Twelver Shia Islam the official religion, largely to distinguish itself from the Sunni Ottoman Empire next door.

That history is a reminder that the map has shifted before, and that rulers have often had reasons of their own for sharpening the line between the two.

The wars and rivalries that make headlines, between Iran and Saudi Arabia, in Iraq after 2003, in Syria and Yemen, are usually described as Sunni versus Shia, and the label is rarely the whole explanation. They are contests over territory, oil, state power and regional influence in which religious identity marks the sides and is used to recruit for them. Where the two communities have lived without a state or a militia making use of the difference, they have mostly lived peacefully. The points to hold together:

• •Shared: the Quran, the Prophet, the five pillars, the core beliefs
• •The origin of the split: who should lead after 632, not what to believe
• •The key Shia idea: divinely guided imams from the Prophet's family
• •The key Sunni idea: leadership by consensus, authority with scholars
• •Modern conflicts: political rivalries wearing a religious label

Sunni and Shia Islam parted over who should succeed the Prophet, and the Shia belief in a line of divinely guided imams is the difference that all the others grew from. The two branches share almost everything else, and the conflicts between them today are far more about power than about theology.