The sectarian divide in the Muslim world is almost as old as the religion of Islam itself. The split occurred after the death of the Prophet Muhammad in the early 7th century, when his Muslim followers split over the question of who should succeed the prophet and lead the new Islamic state he had created on the Arabian Peninsula. The larger group believed Abu Bakr, a companion of Muhammad who was also his father in law, was the prophet’s rightful successor, while a minority believed the prophet’s son-in-law and cousin, Ali, should take on that leadership role. While both sides had evidence to support their claim, Abu Bakr was ultimately appointed first caliph. His followers are now known as Sunni Muslims, while supporters of Ali are known as Shiite Muslims.

Widening of the Divide

The division between Sunni and Shiite Muslims was cemented with death of Ali’s son Hussein in 680, "The Economist" reports. Hussein was killed in Karbala (modern day Iraq) by the ruling Sunni caliph’s troops, who Hussein reportedly would not recognize as caliph. Despite being outnumbered, Hussein and his followers refused to pay allegiance to the caliph and ultimately died in battle. Shiite Muslims believe Hussein sacrificed his life for the survival of Shiite Islam, according to the BBC, which reports the event is central to the Shiite belief in martyrdom.

Justification for Leadership

Sunnis argue that the prophet chose Abu Bakr to lead prayers as he lay on his deathbed, a move they say indicated Muhammad named him as Islam’s next leader. However, Shiites claim that Muhammad, after his last pilgrimage, stood before his companions and named Ali as the “spiritual guide and master of all believers,” the BBC reports. The term Sunni translates to “one who follows the Sunnah" (what the prophet said and did) while Shiite, or Shia, is a combination of the phrase “Shiat Ali” or “partisans of Ali.”

Differences in Theology

All Muslims believe Allah is the only God and that Muhammad was his messenger and final prophet. While Sunni and Shiite Muslims both abide by the teachings of the Quran, over time Shiite Muslims have began to show a preference for certain hadiths (the recorded sayings of the Prophet Muhammad) and Sunnah texts, the BBC reports. The Shiites give preference to texts credited directly to the prophet's family and close associates, while the Sunni consider all sayings in the Hadith and Sunnah equally important. While all Muslims are required to pray five times a day, Shiite Muslims often group those into three daily prayers. Shiites are also identifiable by the small tablet of clay (often from Karbala, the site of Hussein's death) used during prayer, which they rest their forehead on during prostration.

Modern Demographics

Almost 90 percent of practicing Muslims in the world are Sunni Muslims, according to the Pew Research Center. Most Shiite Muslims (between 68 percent to 80 percent, as of 2009) live in just four countries: Iran, India, Iraq and Pakistan. Shiite Muslims make up about 90 percent of the population of Iran. About one-fifth of the world’s Muslims live in a country where Islam is not the major religion.