VATICAN CITY (AFP) - After the pope's resignation on Thursday the Catholic Church will enter a period known as "Sede Vacante" ("Vacant Seat") in which a cardinal takes over interim powers before a new pope is elected.
The cardinal, referred to as the "Camerlengo" ("Chamberlain"), in this case will be Italy's Tarcisio Bertone - a powerful prelate whose handling of Church affairs in recent years has been hugely divisive within the Vatican hierarchy.
The Camerlengo has traditionally had the role of officially certifying the death of a pope - he used to do so by striking the pontiff's forehead on his deathbed with a special silver hammer and calling out the words "Holy Father".
He is also the Vatican official charged with destroying the pontiff's "Fisherman's Ring" - a gold signet ring - in order to prevent the use of the official seal on any counterfeit documents issued in the pope's name.
In his Apostolic Constitution promulgated in 1996, late pope John Paul II decreed that all senior leaders of the Roman Curia - effectively the government of the Catholic Church - had to resign when the pope steps down.
The 79-year-old Bertone will have to resign his post of Secretary of State - effectively the Vatican's number two job - and remain only as Camerlengo.
The only other official who does not have to resign is the Major Penitentiary, who heads up the Vatican's court for forgiveness of sins.
The post is held by Portuguese cardinal Manuel Monteiro de Castro, 74.
Since both Bertone and Monteiro de Castro are cardinals under the age of 80 they can take part in the conclave of "cardinal electors" to elect a pope.
The Camerlengo can be elected pope: the last one was Italian cardinal Eugenio Pacelli, who became Pius XII - the pontiff during World War II.
The Camerlengo handles the day-to-day administration of the Vatican during the interim and, together with other cardinals, he is the official who fixes a date for the pope's funeral as well as the date for holding the conclave.
Pope Benedict XVI is only the second pope in the Church's 2,000-year history to resign of his own free will, meaning that many of the traditions of the "Sede Vacante" are associated with official mourning and funeral rites.
The Vatican post office even puts out a special set of stamps to mark the "Sede Vacante", as well as envelopes bearing Benedict XVI's image.
During the interregnum, the Camerlengo and the cardinals cannot take any decisions that apply beyond the election of a new pope or impinge on the exclusive prerogatives of the pontiff, like appointing new cardinals.
The Camerlengo takes over interim possession of all the papal properties including the Apostolic Palace in the Vatican where the pope resides and Castel Gandolfo, the papal summer residence outside Rome.
He also organises meetings of cardinals known as "congregations" ahead of the conclave which help identify possible candidates for election.
In cases of a papal death, the conclave has to begin between 15 and 20 days after.
The Vatican's canon lawyers are hard at work this week to determine whether this timeline can be brought forward in the case of a resignation - a novelty in the constitution that may require the signature of the outgoing pope.