VATICAN CITY, Feb 28 (Reuters) - The Roman Catholic Church has an elaborate procedure for choosing a new pope after the old one dies. Pope Benedict's surprise decision to resign changed some of the details but the process remains much the same.
When a pope dies or resigns, the Church enters a period known as "sede vacante" ("empty seat" or "vacant see") when the throne of St Peter is unoccupied.
This period traditionally lasts several weeks as the late pope's body lies in state, his funeral is held and cardinals prepare for the conclave, the closed-door meeting to elect a papal successor.
The rules say the conclave must start between the 15 and 20 days after the beginning of the sede vacante.
This time Benedict changed the rules to let the cardinals decide if they want to start earlier, as there was no period of mourning or funeral.
The cardinal electors are due to meet informally on March 1, the day after the pope leaves, and formally from March 4, to begin discussing when to hold the conclave.
During this period they meet daily in closed-door sessions called 'general congregations' to discuss issues facing the Church, get to know each other and size up potential candidates.
A cardinal known as the camerlengo, or chamberlain, becomes the interim chief of the Church with limited powers. A 'particular congregation' - made up of the camerlengo and three elected cardinal assistants - decides on routine matters relating to Church affairs.
The particular congregation also makes sure the pope's "Fisherman's Ring" and the lead seal under which papal documents are dispatched are broken so they cannot be used by anyone else.
Major decision-making is put on hold and a group of Vatican-based cardinals assume temporary civil power over Vatican City, a 108-acre (43-hectare) sovereign state inside Rome.
The meeting takes place in the Sistine Chapel, the famous Vatican hall whose walls and ceilings are adorned with Michelangelo's frescoes.
A total of 115 cardinal electors are expected to attend the conclave, a name from the Latin words "cum clave" (with a key) to signify that it is shut off from the outside world.
Indonesian Cardinal Julius Darmaatmadja will not attend because of his failing eyesight and Scottish Cardinal Keith O'Brien pulled out after resigning as archbishop of Edinburgh following allegations he behaved in an inappropriate way with priests.
The longest conclave lasted nearly three years between 1268 and 1271. Several have lasted only one day. The one which elected Pope John Paul in 1978 lasted less than three days.
Once the conclave begins, the cardinals will not emerge from the Vatican until a new pope has been elected. They will hold voting sessions in the Sistine Chapel and sleep in the Casa Santa Marta hotel inside the Vatican grounds.
The Case Santa Marta was first used for the 2005 conclave. Before that, cardinals had to sleep on camp beds set up in the Vatican Museum rooms adjoining the Sistine Chapel.
They will not be able to communicate with the outside world. They will not be allowed to use telephones or the internet or read newspapers.
WHITE SMOKE OR BLACK SMOKE?
Except for the first day, when only one ballot is held, the cardinals hold two daily balloting sessions until one candidate has a majority of two thirds plus one. All participants are sworn to secrecy about the voting.
The electors cast their ballots on cards printed with the Latin words: "Eligo in Summum Pontificem" (I choose as Supreme Pontiff) and drop them into a large gold chalice.
If the necessary majority has not been reached, the cards and the tally sheets are placed in a stove and burned with an additive to produce black smoke, showing the outside world that a pope has not yet been chosen.
Catholics crowd into St Peter's Square to watch the smoke pouring from a chimney on top of the Sistine Chapel at the end of each viting session. It is not always easy to tell its colour.
If no result has been reached in three days, the sessions are suspended for a day to allow for prayer and discussion. More ballots are held until a two-thirds majority is reached.
WE HAVE A POPE
Once a new pope has been elected, he is asked if he accepts and by which name he wishes to be known. The ballots are burned with an additive to produce white smoke.
In 2005, the Vatican decided to ring the great bell of St Peter's Basilica as an additional sign that the pope had been chosen. But confusion among the people who were supposed to ring it meant the bell lagged the smoke by about 15 munutes.
Inside, the pope dons his new papal vestments - tailors keep large, medium and small sizes ready - and sits on a throne in the Sistine Chapel to receive the other cardinals who file up to pay homage and swear obedience to the Church's new leader.
The senior cardinal deacon then steps out onto the central balcony of St Peter's Basilica overlooking the square and announces in Latin: "Annuntio vobis gaudium magnum. Habemus Papam" (I announce to you a great joy. We have a pope) and reveals the cardinal's name and the name he has chosen as pope.
The new pope then appears on the balcony to deliver his first public pontifical greeting and bless the crowds in St Peter's Square.
A few days later, the new pope celebrates a Mass that marks the beginning of his ministry. (Reporting By Tom Heneghan; Editing by Philip Pullella and Andrew Heavens)