4 - Reaching a decision
The election of a successor for John Paul II will be the first to take place under revised rules.
Many of the elaborate rituals of the conclave were set down by Pope Paul VI in 1975. It was he who decided to exclude cardinals aged over 80 from the voting. He also made it clear that anyone who violated the secrecy of the conclave risked excommunication.
John Paul II changed the rules in 1996. Previously, a candidate had to secure a majority of two-thirds plus one to be elected pope.
Under the new rules, in the initial stages of the conclave, a two-thirds majority is still necessary – with an additional vote if the number of electors is not exactly divisible by three.
But there comes a point after several days of deadlock (after about 30 ballots) when the cardinals can decide to change the procedure to allow a candidate to be elected by winning more than half the votes.
This raises some intriguing possibilities. In the past, the two-thirds rule encouraged cardinals to reach consensus. As previous elections showed, a compromise candidate would emerge if it became obvious that the early favourites could not secure the support of two-thirds of their colleagues.
John Paul II was elected precisely because of this requirement. But under the new rules, if a candidate has the support of more than half the conclave at an early stage, yet falls short of a two-thirds majority, his supporters can decide to hold out until his election becomes possible with only half the vote.
If after three days of balloting nobody has gained the two-thirds majority, voting is suspended for a maximum of one day to allow a pause for prayer, informal discussion and what is described as "a brief spiritual exhortation" by the senior cardinal in the Order of Deacons.
Voting then continues for three sets of seven ballots, with a pause for prayer, discussion and an exhortation by a senior cardinal between each set, until a candidate gains the required majority.
If after this the conclave has still failed to elect a pope, the camerlengo, or chamberlain, invites the cardinals to say how they want to proceed. The requirement for a two-thirds majority can now be dropped - provided more than half the cardinals agree - and the election is then decided by the wishes of the majority.
The cardinals can also choose to restrict the vote to the two candidates who received most votes in the preceding ballot. In this case, a cardinal who wins just over half the votes is now elected.
At the end of the election, a document is drawn up giving the results of the voting at each session, and handed over to the new pope. It is kept in an archive in a sealed envelope, which can be opened only on the orders of the pope.
The only clue about what is going on inside the Sistine Chapel is the smoke that emerges twice a day from burning the ballot papers. Black signals failure. The traditional white smoke, which means a new pope has been chosen, will this year be accompanied by the ringing of the bells of St Peter's Basilica.