Custodian of the Two Holy Mosques in the context of "The Muslim 500"

Masjid al-Haram (Arabic: المسجد الحرام, romanized: al-Masjid al-Ḥarām, lit. 'The Sacred Mosque'), also known as the Sacred Mosque or the Great Mosque of Mecca, is considered to be the most significant mosque in Islam. It encloses the vicinity of the Kaaba in Mecca, in the western region of Saudi Arabia. It is among the pilgrimage sites associated with the Hajj, which every Muslim must perform at least once in their lives if able. It is also the main site for the performance of ʿUmrah, the lesser pilgrimage that can be undertaken any time of the year. The rites of both pilgrimages include circumambulating the Kaaba within the mosque. The Great Mosque contains other important sites, such as the Black Stone, the Zamzam Well, Maqam Ibrahim, and the hills of Safa and Marwa.

As of 2025, the Great Mosque is both the largest mosque in the world, and the most expensive building in the world. It has undergone major renovations and expansions through the years. It has passed through the control of various caliphs, sultans and kings, and is now under the control of the King of Saudi Arabia who is titled the Custodian of the Two Holy Mosques.

The Prophet's Mosque (Arabic: ٱلْمَسْجِد ٱلنَّبَوِي‎, romanized: al-Masjid al-Nabawī, lit. 'Mosque of the Prophet') is the second mosque built by the Islamic prophet Muhammad in Medina, after the Quba Mosque, as well as the second largest mosque and holiest site in Islam, after the Masjid al-Haram in Mecca, in the Saudi region of the Hejaz. The mosque is located at the heart of Medina, and is a major site of pilgrimage that falls under the purview of the Custodian of the Two Holy Mosques (a royal style used by the King of Saudi Arabia.)

Muhammad himself was involved in the construction of the mosque. At the time, the mosque's land belonged to two young orphans, Sahl and Suhayl, and when they learned that Muhammad wished to acquire their land to erect a mosque, they went to Muhammad and offered the land to him as a gift; Muhammad insisted on paying a price for the land because they were orphaned children. The price agreed upon was paid by Abu Ayyub al-Ansari, who thus became the endower or donor (Arabic: وَاقِف, romanized: wāqif) of the mosque, on behalf or in favor of Muhammad. al-Ansari also accommodated Muhammad upon his arrival at Medina in 622.

The king of Saudi Arabia, officially the king of the Kingdom of Saudi Arabia (Arabic: ملك المملكة العربية السعودية, romanized: Malik al-Mamlakat al-ʿArabiyat as-Suʿūdiyya), is head of state of the Kingdom of Saudi Arabia, who holds absolute power. He is the head of the Saudi Arabian royal family, the House of Saud. The king is the commander-in-chief of the Saudi Arabian Armed Forces and the head of the Saudi national honors system. The king is called the "Custodian of the Two Holy Mosques" (خادم الحرمين الشريفين; Khadim al-Haramayn aš-Šarīfayn), a title that signifies Saudi Arabia's jurisdiction over the mosques of Masjid al-Haram in Mecca and Al-Masjid an-Nabawi in Medina. The title has been used many times through the history of Islam. The first Saudi king to use the title was Faisal; however, King Khalid did not use the title after him. In 1986, King Fahd replaced "His Majesty" with the title of Custodian of the Two Holy Mosques, and it has been since used by both King Abdullah and King Salman. The king has been named the most powerful and influential Muslim and Arab leader in the world according to The Muslim 500.