RIYADH To outsiders, the al-Saud ruling family's succession process often appears opaque. But behind the ornate doors of Riyadh's palaces, the senior princes in a family thousands strong have long planned the next step in a complex dance of power.
Saudi Arabia's Defence Minister Prince Salman bin Abdulaziz is widely seen as the next most senior prince in the world's top oil exporter after the death of Crown Prince Nayef, which was reported on Saturday, just eight months after he had become the heir himself.
Unlike in European monarchies, the line of royal succession in Saudi Arabia does not move directly from father to eldest son, but is passed down a line of brothers born to the kingdom's founder King Abdulaziz Ibn Saud, who died in 1953.
So far, five brothers have become kings and around 20 are still alive.
But only a few of those are thought to be realistic candidates to rule the country where Islam was born some 14 centuries ago. Some have already been passed over or renounced their claims to rule.
"The inner circle of the al-Saud (family) can and do exclude from succession those found lacking in lineage, leadership and personal character," said an American embassy assessment of the succession process as detailed in a 1995 diplomatic cable released by WikiLeaks.
Under rules drawn up soon after Abdullah, the current monarch, became king in 2005, succession decisions lie with an "allegiance council" of the ruling al-Saud family.
When a king dies, his crown prince accedes to power and nominates a new crown prince.
The allegiance council, which has 34 princely members who each represent the family of a son of Ibn Saud, votes to approve the king's choice and has the power to elect its own nominee.
The council swore allegiance to Nayef as crown prince after the death of his predecessor Prince Sultan last year, but it was unclear if it actually voted.
While the next stage of the power planning transfer process appears straightforward and uncontroversial, there will be keen interest in how the council handles more complex succession decisions in the future.
To avert a repeat of the situation that unfolded after King Fahd fell ill, when Abdullah's leadership was undermined by his ambiguous position as de facto regent without the formal title, the council has the power to remove a king or crown prince who is too sick to rule.
Given that Abdullah is nearly 90, the clause might prove an important failsafe in a country where power emanates from the top.
The next prince presumed to be in line for power is Prince Salman, who was made defence minister in November after Sultan's death, cementing his role near the top of the family hierarchy.
He served for five decades as governor of Riyadh province, a role that made him responsible for settling disputes between other members of the ruling family.
An owner of several major media outlets that have pushed a pro-government position and supported King Abdullah's cautious reforms, he said in 2007 that social change in the kingdom had to be slow and that the country was not ready for democracy, according to a U.S. embassy cable released by WikiLeaks.
Because Ibn Saud had so many sons born over a period of nearly five decades, various groups of princes have envisaged different paths for the kingdom's future.
The most powerful bloc within the variegated ranks of the al-Saud family is thought to comprise the sons born to Ibn Saud by his favorite wife Hassa bint Ahmed al-Sudairi: the so-called "Sudairi Seven".
They included the late King Fahd, Prince Sultan, Prince Nayef and Prince Salman. Three other brothers include a former defence minister, a former deputy defence minister and a deputy interior minister.
To many analysts, the key question is what will happen when the succession moves beyond the sons of Ibn Saud to one of his grandsons.
That decision might not be made for some years, but outside observers already see the emergence of a handful of contenders who appear better qualified to rule than their cousins.
There are no formal rules to dictate how the generational transition will be made other than through Abdullah's allegiance council. But any candidate would need broad support among the family as well as a strong record of political experience.
That might point to one of the Sudairis, such as Fahd's son Mohammed, who is governor of the Eastern Province, Sultan's son Khaled, who is deputy defence minister, or Nayef's son Mohammed, who as deputy interior minister was partly responsible for the successful suppression of an al Qaeda uprising six years ago.
Salman's son Sultan is the tourism minister and was the first Arab astronaut.
Another potential candidate among the third generation is Prince Khaled al-Faisal, son of the former King Faisal and the well regarded governor of Mecca Province, one of the most prestigious jobs in the country.
Prince Miteb bin Abdullah, the son of the present king, has inherited his father's position as head of the Saudi Arabian National Guard, a military unit that is separate from the ordinary armed forces whose role is to protect the kingdom from coups d'etats.
(Editing by Andrew Osborn)