Most monarchies favour primogeniture, a simple way of passing the crown from one generation to the next. Kingship in Muslim dynasties has tended instead to pass between brothers -- but whose son should then inherit the throne?
Ottoman sultans solved this problem by murdering their brothers. That is not easy, though, if you happen to have 45-odd male siblings, as was the case for the five succeeding sons of Abdel Aziz bin Saud, the founder of Saudi Arabia, who have ruled since his death in 1953.
Most of those sons are now dead, leaving Saudis to wonder when the prolonged hold of this gnarled second generation will end. On March 27, the reigning monarch, King Abdullah, thought to be at least 89, provided an answer: not soon.
A royal decree appointed his youngest surviving brother, Muqrin, born in 1945, as second in line to the throne after the crown prince, Salman, who is 78 and ailing. Should the newly anointed heir survive as long as Abdullah, he could still be king in 2034.
A former intelligence chief, former governor of the holy city of Medina and a pilot who trained at a Royal Air Force college in Britain, Muqrin is considered a steady hand, though palace gossips sniff that his mother was a Yemeni concubine.
He is close to the king, though, and well thought of, says Joseph Kechichian, author of Succession in Saudi Arabia (Palgrave Macmillan, 2001).
At a function in Riyadh days after the announcement, Kechichian adds, the prince showed himself to be "dynamic and congenial."
The appointment of a second heir prompted whispers that Abdullah might soon abdicate. That is unlikely. Saudi kings tend to rule until death, though one was ousted by a brother.
More likely, Abdullah fears that Salman, believed to be suffering from Alzheimer's, will be unable to take over. The statement from the royal palace hinted as much, referring to a situation in which the positions of both king and crown prince could become "vacant."
The House of Saud can ill afford for that to happen, since the oil-rich kingdom faces mounting challenges.
The ruling coterie fears the Arab spring might yet provoke its youthful, Internet-using population of 30 million into more forcefully airing grievances such as the strictness of Islamic laws and the lack of jobs.
They fear that Saudi jihadists who have gone to fight in Syria could return to make trouble at home.
They especially dread developments abroad. In particular, the royal Saudis are "hysterical," in the word of a recent visitor, over America's outreach to Iran.
They have fallen out with Qatar over its support for the Muslim Brotherhood and, as chief patrons of Syria's rebels, they have failed to create a force strong enough to turn the tide against President Bashar Assad.
Some Saudis quietly criticize Abdullah for once again ducking the challenge of picking an heir from the next generation. That now most likely will be Muqrin's choice if he wins the crown and lasts into old age.
The third generation, the founder's grandsons, now numbers hundreds of princes, with subsequent generations probably taking the male tally past 8,000, of whom at least a score might consider themselves eligible for the throne one day.
They might be getting impatient.