Here are the rules in use to elect the 266th pope:
WHO VOTES? Only cardinals under age 80 are eligible; in this case 115 men fit the bill and will vote.
Two cardinals who were eligible stayed home: The emeritus archbishop of Jakarta, Cardinal Julius Darmaatjadja, who is ill, and Scottish Cardinal Keith O'Brien, who recused himself after admitting to inappropriate sexual behavior.
WHAT IS THE RITUAL?
The conclave's first day begins with the "Pro eligendo Romano Pontificie" Mass for the election of a pope. (right)
In the afternoon, cardinals gather in the Pauline Chapel of the Apostolic Palace and file into the Sistine Chapel chanting the Litany of Saints and the Latin hymn "Veni Creator," imploring saints and the Holy Spirit to help them pick a pope
Standing under Michelangelo's "Creation" and before his "Last Judgment," each cardinal places his hand on a book of the Gospels and pledges "with the greatest fidelity" never to reveal the details of the conclave.
A meditation on the qualities needed for the next pope and the challenges ahead for the church is delivered by Maltese Cardinal Prosper Grech.
The master of liturgical celebrations then cries "Extra omnes," Latin for "all out." Everyone except the cardinals leaves and the voting can begin.
HOW DO THEY VOTE? Each cardinal writes his choice on a paper inscribed with the words "Eligo in summen pontificem," or "I elect as Supreme Pontiff." They approach the altar one by one and say: "I call as my witness, Christ the Lord who will be my judge, that my vote is given to the one who, before God, I think should be elected."
The folded ballot is placed on a round plate and slid into an oval silver and gold urn.(pictured L)
In the past, a single chalice was used to hold the ballots. But conclave changes made by Pope John Paul II in 1996 required three vessels: one for chapel ballots, another for ailing cardinals at the Vatican who can vote from their beds and the third to hold the ballots after counting. No cardinals are expected to require the bedside voting, but all three flying saucer-shaped urns were in the Sistine Chapel regardless.
Once cast, the ballots are opened one by one by three different "scrutineers," who note the names down and read them aloud. Cardinals can keep their own tally on a sheet of paper provided but must turn their notes in to be burned at the end of voting.
The scrutineers then add up the results of each round of balloting and write the results down on a separate sheet of paper which is preserved in the papal archives.
As the scrutineer reads out each name, he pierces each ballot with a needle through the word "Eligo" and binds the ballots with thread and ties a knot. The ballots are then put aside and burned in the chapel stove (pictured right) along with a chemical to produce either black or white smoke.
If no one is elected after three days - by Friday afternoon - voting pauses for up to one day. Voting resumes and if no pope is elected after another seven ballots, there is another pause, and so on until about 12 days of balloting have passed.
Under norms introduced by Benedict XVI just before he resigned, the cardinals then go to a runoff of the top two vote-getters. A two-thirds majority is required; neither of the two top candidates casts a ballot in the runoff.
WHAT HAPPENS ONCE THE POPE IS ELECTED? Once a cardinal has been elected pope, the master of liturgical ceremonies enters the Sistine Chapel and the senior cardinal asks "Do you accept your canonical election as Supreme Pontiff?" Assuming the cardinal says "I accept," the senior cardinal then asks: "By what name do you wish to be called?" The master of liturgical ceremonies, Monsignor Guido Marini, then enters the information on a formal document.
At this point, white smoke pours out of the Sistine Chapel chimney and bells of St. Peters toll.
The new pope then changes into his papal white cassock, and one-by-one the cardinals approach him to swear their obedience.
In a change for this conclave, the new pope will stop and pray in the Pauline Chapel for a few minutes before emerging on the loggia of the balcony overlooking St. Peter's Square (left).
Preceding him to the balcony is French Cardinal Jean-Louis Tauran, the protodeacon, who announces "Habemus Papam!" Latin for "We have a pope" and then introduces him to the world in Latin.
The new pope then emerges and delivers his first public words as pope.
FAMOUS FIRST WORDS?
Pope John Paul II charmed the crowd of thousands on Oct. 16, 1978 when he first emerged on the loggia, no easy task given his predecessor had only lived as pope for 33 days and Karol Wojtyla was the first non-Italian elected in 455 years.
Noting that he came from a far-away land - Poland - he told the crowd that he would speak in their ("our") language.
"If I make a mistake, you will correct me," he said to cheers.
Retired Pope Benedict XVI offered a similarly modest gesture on April 19, 2005, telling the crowd he was but a simple "humble worker in the vineyard of the Lord."
The moment Cardinal Albino Luciani learned his colleagues had elected him pope, he responded: "May God forgive you for what you've done."
The remark, by the man who became Pope John Paul I, was seen as an expression of humility - but also a commentary on the mammoth task ahead.