The successor to Pope John Paul II can choose a Latinised form of his Christian name or the name of an earlier Pope, or a saint's name.
He may also want to pick a name that would attribute a quality to him, such as pius (pious) or innocent.
Some allow personal considerations to guide them, such as Pope John XXII, who chose his name to honour the memory of his father.
Up to the end of the first millennium, Popes generally kept their Christian name.
The first to change his handle was John ii in 533. In 1009, Sergius IV, whose given name was Peter, switched out of respect to Saint Peter, feeling that becoming Peter II was too presumptuous.
The first Pope name to be repeated through the history of the church was Sixtus. The first Sixtus took the pontificate in the second century and to date there have been four others.
The given names most often chosen have been John (23 times), Gregory (16), Clement (14), Innocent (13), Leo (13) and Pius (12).
As with kings and emperors, Popes add a roman numeral to their name if they had a namesake as predecessor.
The first Pope to use a double name was John Paul I in 1978 who wanted to pay tribute to his two immediate predecessors, John XXIII and Paul VI.
At the end of John Paul I's one-month long pontificate in September 1978, his successor, Polish Cardinal Karol Wojtyla, chose the same name.