Custodian of the Two Holy Mosques

Custodian of the Two Holy Mosques (abbreviation CTHM; خَادِمُ ٱلْحَرَمَيْنِ ٱلشَّرِيفَيْنِ), or Protector of the Two Holy Cities, is a royal style that has been used officially by the monarchs of Saudi Arabia since 1986. The title has historically been used by many Muslim rulers in the past, including the Ayyubids, the Mamluks, the Ottomans and the Sharifain rulers of Hejaz. The title was sometimes regarded to denote the de facto Caliph of Islam, but it mainly refers to the ruler taking the responsibility of guarding and maintaining the two holiest mosques in Islam: Al-Haram Mosque (اَلْمَسْجِدُ ٱلْحَرَامُ, "The Sacred Mosque") in Mecca and the Prophet's Mosque (اَلْمَسْجِدُ ٱلنَّبَوِيُّ) in Medina, both of which are in the Hejazi region of Saudi Arabia. The Custodian has been named the most powerful and influential person in Islam and the Sunni branch of Islam by The Muslim 500, as well as the most powerful Muslim and Arab ruler in the world.

History
It is believed that the first person to use the title was Saladin.

After defeating the Mamluks and gaining control of the Mecca and Medina in 1517, the Ottoman sultan Selim I adopted the title. Rather than style himself the Ḥākimü'l-Ḥaremeyn (حَاكِمُ الْحَرَمَيْن, Ruler of the Two Sanctuaries), he accepted the title Ḫādimü'l-Ḥaremeyn (خَادِمُ الْحَرَمَيْن, Servant of the Two Sanctuaries).

The first King of Saudi Arabia to assume the title was Faisal bin Abdul Aziz (1906–1975). His successor Khalid did not use the title, but the latter's successor Fahd did, replacing the term "His Majesty" with it. The current king, Salman bin Abdulaziz Al Saud, took the same title after the death of King Abdullah, his half brother, on 23 January 2015.