Beirut, Lebanon: Tensions between Saudi Arabia and Iran have reached new heights with Riyadh’s execution on Saturday of a Shia cleric despite Tehran’s calls for clemency.

Here are some questions and answers about the religious split dividing Sunni Saudi Arabia and Shia Iran:

Q: What is the split about?

A: The split emerged over a disagreement on who should succeed the Prophet Mohammed, who died in 632.

Some felt his successor should be chosen from among his followers, while others—the “Shia Ali” or Partisans of Ali—believed the position should stay within the prophet’s family and that Mohammed had backed his cousin Ali for the post.

Eventually, Abu Bakr, a confidant of the prophet, became the first caliph, and was succeeded by two other members of Mohammed’s inner circle before Ali assumed the post.

Shia Muslims hold all of the prophet’s family, and particularly Ali and his sons Hassan and Hussein, in high esteem.

Hussein’s murder in 680 along with most of his family is commemorated each year by Shia Muslims in the Ashura ritual, a day of mourning.

Q: What are the religious differences?

A: Sunni and Shia Muslims share many practices and beliefs, including the so-called five pillars of Islam that encompass fundamentals such as five daily prayers and the pilgrimage to Makka.

Shia Muslims, however, consider Ali to be the first of a series of imams, who are successors to the prophet and provide models and guidance for the faithful.

As a result, Shia Islam has a stricter clerical hierarchy, exemplified with the ayatollahs and imams of Iran and Iraq, than does Sunni Islam.

Shias believe in an unbroken line of 12 imams who succeeded the prophet, beginning with Ali, and that the twelfth imam is in occultation but will return to restore justice on earth before the Day of Judgment.

While Sunni Muslims deem the four men who succeeded Mohammed to be the “rightly guided” caliphs, they do not place any particular significance on the religious practices of the leaders who came after them.

Shia Muslims also incorporate mention of Ali into the Muslim shahada, or profession of faith, and have several religious holidays distinct from Sunnis that are based on their reverence for Ahl al-Bayt, or the family of the prophet.

These include Ashura and Arbaeen, which commemorate the deaths of Ali’s son Hussein and the suffering of his surviving family members respectively.

They also go on pilgrimages to several holy sites specific to Shias, in addition to the Haj pilgrimage to Makka in Saudi Arabia.

Q: What are the demographics?

A: According to a 2015 study by the Pew Forum, there are just under 1.6 billion Muslims worldwide.

Sunnis are generally thought to constitute up to 90% of the population of Muslims, with the rest divided between different Shia branches.

The balance is less stark in the Middle East because of large Shia populations in countries such as Iran, with some 80 million adherents to the branch.

Iran is traditionally regarded as the most powerful and important Shia-majority country in the world, with Shias also forming a majority in neighbouring Iraq and Bahrain.

There are large Shia populations elsewhere, including in Yemen, Pakistan and Lebanon. Offshoots and branches of Shia Islam are present in other countries too, including Syria and India.

Saudi Arabia, home of the pilgrimage sites of Makka and Medina, is often regarded as the centre of Sunni Islam and the ruling king is known as “custodian of the two holy mosques” in reference to mosques at the two sites.

The Al-Azhar institution in Egypt is often regarded as Sunni Islam’s most important academic institution and a reference for the faithful.