The Sunni-Shi'a divide emerged following the death in 632 of the Prophet Muhammad amid competing views over who should succeed him -- the Prophet's trusted companion, Abu Bakr, or Muhammad's cousin and son-in-law, Ali. Over time, these groups came to be known as Sunnis -- from "sunnah," the sayings, actions, and teachings of Muhammad -- and Shi'a, derived from the phrase meaning "partisans of Ali." As many as 90 percent of the world's some 1.6 billion Muslims are Sunni. Most Shi'a live in Iran, Iraq, and pockets of other countries in the Middle East and beyond.