The gravity of the Pope's latest medical bulletins have inevitably turned attention to the nature of the process by which his successor is chosen, an ancient and convoluted mechanism.
The College of Cardinals elects the new Pope in conclave, which is the process of sequestering the voting members of the college in Vatican City so that they have no contact with the outside world. The word "conclave" comes from the Latin phrase "cum clavis", meaning "with key".
The term is suitable since the cardinals are locked inside the Sistine Chapel in the Apostolic Palace during the voting.
A conclave begins no earlier than 15 days and no later than 20 days after the Pope's death.
Cardinals participating in conclave stay in St Martha's House, a hospice inside the Vatican that has 130 rooms. Arrangements are made to ensure that the cardinals are not approached as they are transported between St Martha's and the Sistine Chapel.
Pope John Paul II himself described the complex procedures that will be used to elect the 265th successor to St Peter in Universi Dominici Gregis (UDG), an Apostolic Constitution issued by the Pope in 1996. It is an accepted practice for popes to publish the norms that regulate the election of their successors, and they often make small adjustments to the procedures.
John Paul II explained that these changes are made "with the intention of responding to the needs of the particular historical moment".
According to the UDG, the current rules for electing a new pope are:
The maximum number of electors from the College of Cardinals is 120. The college is currently composed of 194 cardinals.
Any cardinal who turns 80 before the day the Papacy is vacated, either by death or resignation, cannot take part in the election.
Currently, 135 cardinals are eligible to vote under this rule (15 of those 135 would be disqualified from the vote because the limit is 120).
A two-thirds-plus-one majority is required to elect a pope.
Two ballots each are held in the morning and afternoon, for a total of four a day.
If a new pope is not selected after 12 to 13 days, the cardinals may choose to impose a majority vote, which would allow selection of a new pope by a simple majority.
Each rectangular ballot is inscribed at the top with the words Eligo in Summum Pontificem, meaning "I elect as supreme pontiff". Below these words, each cardinal writes down the name of the person he chooses as the Pope.
The vote is done in secret with paper and pen.
The voting cardinal then folds the ballot twice, holds it in the air, and carries it the chapel's altar.
The cardinal places the ballot on a plate that sits atop the ballot receptacle and uses the plate to drop the ballot into the receptacle.
Three scrutineers, who are selected by all of the cardinals, are charged with counting the ballots. Once the ballots are collected, the scrutineers count the ballots to determine if everyone has voted. If the number of ballots doesn't match the number of electors, the ballots are immediately burned and another vote is taken.
The steps for the vote-tallying procedure are:
The first scrutineer takes a ballot, notes the name on it, and passes it to the next scrutineer.
The second scrutineer notes the name and passes it to the third scrutineer.
The third scrutineer reads aloud the name on the ballot, pierces the ballot with a needle through the word Eligo at the top of the ballot, and slides the ballot on to a string of thread.
Once all ballots are read, the scrutineers write down the official count on a separate sheet of paper.
The third scrutineer ties the ends of the thread on which the ballots are placed in a knot to preserve the vote.
The ballots are placed in a receptacle.
After each vote, the ballots and any notes regarding them are burned. Smoke from the burning of the ballots appears over the Vatican Palace.
If no pope has been chosen, a chemical is applied to the ballots to create black smoke when burned. White smoke signals that the Pope has been elected.
The newly elected Pope remains for life, or until he retires.
Top contenders diverse group
Papal scholar John-Peter Pham has picked five leading contenders to succeed Pope John Paul II.
He believes the next Pope must have good health, be prepared to travel, have intellectual clout, and be able to speak to the world's 1.1 billion Catholics. Hence the need to be multilingual - ideally in Italian, Latin, English, Spanish, French and German. Plus he needs to have pastoral experience. His top five are:
Dionigi Tettamanzi, 71, Italy - the Archbishop of Milan, is the frontrunner. Tettamanzi is a pastor and an intellectual and as someone close to John Paul II, insiders say he he represents continuity, but with new ideas. Tettamanzi can count on the support of Cardinal Giovanni Battista Re, who heads the bishops' congregation, and Camillo Ruini, head of the Italian Church. He's also close to Opus Dei, the ultraconservative Catholic group.
Francis Arinze 73, Nigeria - Archbishop Emeritus of Onitsha, Nigeria, pro-president of the secretariat for Non-Christians. In 2002, after serving as the head of the Pontifical Council for Interreligious Dialogue, he was named the head of the Congregation for Divine Worship and Discipline of Sacraments. Arinze is a conservative who takes a hardline position on abortion and contraception and denounces homosexuality.
Christoph Schonborn, 60, Austria - Archbishop of Vienna. Schonborn is titled aristocracy related to every European royal family. Also a brilliant theologian with sensitivity to the Christian East, orthodoxy and Eastern bloc Catholics.
But his relatively young age and that he comes from the German camp could work against him.
Angelo Scola, 64, Italy - the Patriarch of Venice. A scholar and a moderate, Scola is likely to have the backing of Opus Dei.
Severino Poletto, 72, Italy - the Archbishop of Turin. Poletto was named Bishop of Asti last year. His prospects derive from his spiritual and pastoral qualities and his links to Vatican secretary of state, Angelo Sodano.
But an article in BusinessWeek - Why The Next Pope May Be A Surprise - puts forward a number of other possibilities. Mumbai's archbishop Ivan Dias, a friend of Mother Teresa, is seen as an outside possibility. He does does, however, speak 16 languages.
Honduran Oscar Rodriguez Maradiaga, speaks Spanish, near-perfect English and Italian, and decent French, Portuguese, German and Greek. He teamed with Bono to campaign against third world debt and is known for his work with the poor. Latin Americans make up more than 50 per cent of all Catholics.
Another Third World favourite is Cardinal Claudio Hummes of Brazil, a moderate who has spoken out on human rights issues.
Seventy-one-year-old Cardinal Godfried Danneels of Brussels is also a possibility, but he is thought too liberal.
Joseph Ratzinger 78, is seen as a possible transitional Pope - a German who has been John Paul's enforcer on Church doctrine and is a social conservative.