The shock resignation of Pope Benedict XVI has raised questions about how the leader of the Catholic Church is able to step down from office.
No Pope has resigned in modern times, and the last one to leave the post was Pope Gregory XII in 1415 - 598 years ago.
The 85-year-old Pontiff said he would resign at the end of February due to his 'incapacity to adequately fulfill the ministry entrusted to me.'
Under Canon Law of the Catholic Church, the only conditions for the Pope resigning are that it is done freely and is properly published.
The Pope's spokesperson Federico Lombardi said on Monday that Benedict's resignation was 'in full compliance with church law', and that he wasn't resigning due to any 'difficulties in the papacy'.
But Pope Benedict's decision to step down was seen as all the more unusual given that most incumbents die in office.
The only occasion in recent memory which was even vaguely similar came during the Second World War.
At the time there was speculation that Pope Pius XII drew up a document saying that if he was kidnapped by the Nazis, it should be considered that he had resigned and a successor should be found.
Pope Gregory XII resigned in 1415 during the Western Schism, which threatened to break up Roman Catholicism.
Two rivals, Avignon Pope Benedict XIII and Antipope John XXIII, had also made claim to the post.
Gregory was elected in 1406, but the division saw him state that if Benedict renounced his claim to the position, he would renounce his.
Negotiations took place between the three, but cardinals at the Council of Pisa eventually rejected both their claims and chose Alexander V.
Pope Gregory resigned in 1415, in a move which is said to have united Catholic Church.
Pope Benedict will step down after almost eight years, having been elected in April 2005.
Related: Pope Benedict XVI resigns: Live blog
The last post on Pope Benedict's Twitter feed on Sunday said: "We must trust in the mighty power of God's mercy.
"We are all sinners, but His grace transforms us and makes us new."