The possibility of the resignation of a reigning Pontiff is recognised in the Code of Canon Law of the Catholic Church.
The Code states, “If it should happen that the Roman Pontiff resigns his office, it is required for validity that he makes the resignation freely and that it be duly manifested, but not that it be accepted by anyone” (Canon 332, §2).
The norms, promulgated in 1996 by John Paul II, for the election of a Roman Pontiff also recognise that a vacancy in the office of the Bishop of Rome can occur not only as a result of the death of a Pope but also by his valid resignation (Universi Dominici Gregis, Part I, Chapter 1, §3 and Chapter 3, §77).
In February 2013, the norms were amended slightly by Benedict XVI’s final Motu Proprio Normas nonnullas.
Following a pope’s resignation, the office of the Roman Pontiff (the Pope) becomes vacant and a new Bishop of Rome needs to be elected. The current Holy Father will at that point cease to be the Bishop of Rome and will no longer hold the title of Pope and will return to the use of his baptismal name, together with the customary mode of address pertaining to a Cardinal Bishop, in all formal modes of address.
During the period of the vacancy in the office of Roman Pontiff, the governance of the Church will be entrusted to the College of Cardinals solely for the dispatch of ordinary business and of matters which cannot be postponed, and for the preparation of everything necessary for the election of a new Pope.
The College of Cardinals cannot, however, exercise any power or jurisdiction which is proper to that office of the Roman Pontiff. The Cardinal Camerlengo of the Holy Roman Church, currently Cardinal Tarcisio Bertone, has the duty of safeguarding and administering the goods and temporal rights of the Holy See.
It is the responsibility of the Dean of the College of Cardinals to convoke the Congregations of the Cardinals for the election.
The current norms determine that a Conclave to elect a new Pope must begin no sooner than 15 days, and no later than 20 days from the commencement of the vacancy (Sede vacante) of the See of the Rome.
The next Pope will be elected by the members of the College of Cardinals who are under the age of 80 when the Conclave begins. Cardinals over the age of 80 can however participate in the discussions of the General Congregations of the Cardinals which are held before the Conclave for the election begins. Cardinal electors and their assistants now reside in the purpose-built Domus Sanctae Marthae, built in 1996 within the Vatican.
The Third Lateran Council in 1179 decreed that a candidate must gain a majority of two-thirds of the votes of the Cardinals to be elected Pope. This was modified by John Paul II in the Apostolic Constitution, Universi Dominici Gregis who determined that if no candidate had been elected after 33 ballots, the Cardinal Camerlengo could invite the Cardinals to decide to elect a Cardinal who got more than half of the votes, that is by an absolute majority. However, this was reversed by Pope Benedict XVI in the Apostolic Letter De aliquibus mutationibus and determined that a Pope can only be elected by a two thirds majority vote no matter how many ballots are necessary.
Six Popes, each under very different historical circumstances, have either resigned or abdicated their office as Roman Pontiff: Pontian (in 235); Silverius (in 537); John XVIII (in 1009); Benedict IX (in 1045); Celestine V (in 1294); and, Gregory XII (in 1415).