King Salman has two other titles as well: "Custodian of the Two Holy Places" and prime minister. This broadens the range of possibilities for transferring responsibilities to MbS. The scenarios could unfold as follows:
Salman abdicates and MbS becomes king. "Abdication" is probably not a favored option in the kingdom. It was last used in 1964 when the spendthrift King Saud was forced to give up after six years of tension with his half-brother Faisal, who replaced him. More recently, in 2013, Emir Hamad al-Thani of Qatar abdicated in favor of his son Tamim but retains much influence, along with the official title of "Father Emir." Given Riyadh's current bad blood with Qatar, the chances of Salman emulating the "Father King" model are likely zero, but a different slice of history could make full abdication more acceptable.
In 1902, Ibn Saud (only twenty-two at the time) led a group of fighters from exile to recapture his family's ancestral village of Dariyah in central Arabia. In response, his father Abdulrahman ceded leadership of the House of Saud to him. Today, King Salman is said to see Ibn Saud's character in his son, and the Wall Street Journal reports that he has already made a video announcing that MbS will be king.
Salman gives up the throne but remains Custodian. Since Ibn Saud captured the holy cities of Mecca and Medina in 1925, successive rulers have taken responsibility for the Islamic shrines. King Fahd formalized this role in 1986, changing his title from "majesty" to "Custodian of the Two Holy Places." Retaining the religious title but relinquishing political leadership would be consistent with the sense that the former is more important -- a key ingredient in Saudi Arabia's claim to leadership of the wider Arab and Muslim worlds.
Salman appoints MbS prime minister. At present, the king is prime minister and the crown prince is deputy prime minister. Yet the weekly meetings of the Council of Ministers, which are chaired by the prime minister, are not the country's most crucial decisionmaking forums. That honor goes to the Council of Political and Security Affairs and the Council of Economic and Development Affairs, two bodies that were created in 2015 and are now chaired by MbS. Administratively, naming MbS as prime minister would arguably be tidier than the current arrangement. But this may be a delicate issue: Faisal and King Saud engaged in a long tug-of-war over bureaucratic control before the former's accession, so Salman would have to be truly willing to give up the job if this division of labor is to work today.
MbS becomes regent. When Salman travels abroad, as he did to Moscow earlier this month, he "deputizes" MbS "to administer the state's affairs and take care of the interests of the people during his absence," according to the Saudi Press Agency. A version of this option -- regency -- is available in circumstances of illness or lengthy medical treatment abroad. Yet a protracted regency could be contentious. After King Fahd suffered a debilitating stroke in late 1995, Crown Prince Abdullah was appointed regent, but he held the title for only a few weeks -- apparently because Fahd's powerful full brothers (Sultan, Nayef, and Salman) were anxious to deny Abdullah complete authority. Despite the king's poor physical condition thereafter, Abdullah did not assume full formal power until his own accession in 2005.
Salman dies. As crown prince, MbS would become king provided his leadership is acknowledged by senior members of the House of Saud, who must give him the oath of allegiance. Yet reported schisms in the royal family could lead some figures to contest his new authority. When Salman made MbS crown prince four months ago, three of the thirty-four princes on the A...