LAGOS Uncertainty over whether Nigerian President Goodluck Jonathan will stand in elections next year is largely due to a nebulous understanding that power rotates between the Muslim north and Christian south.
Although not formally set in writing, there is an agreement among the political elite in the ruling People's Democratic Party (PDP) that the presidency should alternate between north and south after every two four-year terms.
Jonathan, a southerner, took over as head of state earlier this year after the death of President Umaru Yar'Adua, a northern Muslim who was part way through his first term.
Some northern power brokers say what should have been his second term can only be taken by another northerner and that Jonathan should therefore not stand. Other northerners say it is time for the "zoning agreement" to be jettisoned.
Here are some questions and answers about the agreement:
WHAT ARE ITS ORIGINS?
Africa's most populous nation is roughly equally divided between Christians and Muslims. More than 200 distinct ethnic groups generally live peacefully side by side.
But civil war left more than one million people dead between 1967 and 1970 and there have been frequent bouts of ethnic and religious violence, particularly in the central Middle Belt which lies on the fault-line between north and south.
More than 13,500 people have died in religious or ethnic clashes since the end of military rule in 1999, according to U.S.-based Human Rights Watch.
Clashes between Christian and Muslim gangs in the central city of Jos killed hundreds of people earlier this year.
The violence had more to do with rivalry for political and economic power than with religious fervor. Some Jos residents -- mostly members of the largely Muslim Hausa ethnic group -- are classified as "non-indigenes" and denied opportunities given to those classified as original inhabitants, rights groups say.
The notion of sharing power between north and south aims to prevent such disputes becoming a factor in federal politics.
"It has been an unwritten law since 1979," said Abubakar Momoh, politics professor at Lagos State University.
"This is fostered by circumstances and the tragedy of history and there is nothing you can do about that."
HOW FORMAL IS THIS AGREEMENT?
There is no reference in Nigeria's constitution to the idea of the presidency rotating between north and south.
But the constitution and manifesto of the ruling People's Democratic Party (PDP) both make reference to "geo-political balancing" as a fundamental principle of power sharing.
CAN IT BE ENFORCED?
The PDP's manifesto acknowledges the supremacy of the Nigerian constitution, meaning that there is no legal document requiring that presidential candidates should come from any particular geographic region or religious background.
But any attempt to break the spirit of the agreement could cause deep divisions within the ruling party and potentially stoke unrest in parts of the country.
Should Jonathan be persuaded not to stand in 2011, his supporters in his home region of the Niger Delta, the restive heartland of Nigeria's mainstay oil and gas industry, could launch protests.
But should he run with the support of the PDP, this could also stir up resentment among northern youths, who have already mounted small-scale protests to lobby against his candidacy.
(Editing by Giles Elgood)