Sunni and Shia Islam are the two major denominations of Islam. The demographic breakdown between the two denominations is difficult to assess and varies by source, but a good approximation is that 75–90% of the world's Muslims are Sunni and 10–20% are Shia, with most Shias belonging to the Twelver tradition and the rest divided between several other groups.
Sunnis are a majority in most Muslim communities: in South East Asia, China, South Asia, Africa, and most of the Arab world. Shia make up the majority of the population in Iran, Iraq, Lebanon, Azerbaijan, and Bahrain (all together called the Shia Crescent), while Pakistan has the second-largest Shia Muslim (Twelver) population in the world.
The historic background of the Sunni–Shia split lies in the schism that occurred when the Islamic prophet Muhammad died in the year 632, leading to a dispute over succession to Muhammad as a caliph of the Islamic community spread across various parts of the world which led to the Battle of Siffin. Sectarian violence persists to this day from Pakistan to Yemen and is a major element of friction throughout the Middle East.
Over the years, Sunni–Shia relations have been marked by both cooperation and conflict. Tensions between communities can intensify during power struggles, such as the Bahraini uprising, Syrian civil war or the 2003 Iraq War. Today there are differences in religious practice, traditions and customs, often related to jurisprudence. Although all Muslim groups consider the Quran to be divine, Sunni and Shia have different opinions on hadith.