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 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 27th the reigning 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, 78 and ailing. Should the newly anointed heir survive as long as Abdullah, he could still be king in 2034.

A former intelligence chief, governor of the holy city of Medina and 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. But he is close to the king and well thought of, says Joseph Kechichian, author of “Succession in Saudi Arabia”. At a function in Riyadh days after the announcement, the prince showed himself to be “dynamic and congenial”, says Mr Kechichian.

The appointment of a second heir prompted whispers that Abdullah may soon abdicate. That is unlikely. Saudi kings tend to rule until death, though one was ousted by a brother (see chart). 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 where 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 may yet provoke its youthful, internet-using population of 30m into more forcefully airing grievances, such as the strictness of Islamic laws and the lack of jobs. They fear Saudi jihadists who have gone to fight in Syria could return to make trouble at home. And they 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 Brothers. And as chief patrons of Syria’s rebels, they have failed to create a force strong enough to turn the tide against Bashar Assad.

Some Saudis quietly criticise Abdullah for ducking once again the challenge of picking an heir from the next generation. That will now most likely 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; subsequent generations probably take the male tally past 8,000, of whom at least a score may consider themselves eligible one day for the throne. They may be getting impatient.