Shias believe that God always provides a guide, first in the direct descendants of Ali, known as Imams, and then ayatollahs, or experienced Shia scholars who are sought as a source of emulation. The sect splintered due to differences over the proper line of succession among Ali's descendants.
Shia doctrine is based on the teachings of the imams, descendants of Muhammad who were the original and sole interpreters of the Qur'an and Islamic articles of faith. Most Shia adhere to the Ithna 'Ashariyah or “Twelver” tradition, which is the official state religion of Iran.
Most Shias (between 68% and 80%) live in four countries: Iran, Pakistan, India and Iraq. Iran has 66 million to 70 million Shias, or 37-40% of the world's total Shia population. Iraq, India and Pakistan each are home to at least 16 million Shias.