The so-called conclave will choose John Paul II's successor. It is made up of cardinals that have not yet turned 80 -- there are currently 117 of them.

While the cardinals already have a chance to discuss candidates since many of them arrived in Rome ahead of the late pope's funeral, the actual election cannot begin any earlier than 15 days after John Paul II's death nor later than 20 days after his passing. Vatican officials announced Wednesday that cardinals have chosen April 18 as the day to begin deliberations.

The late pope's will will be published on Thursday.

Simple requirements

Theoretically, there's a huge number of people who qualify for the job of pontiff.

"Every Catholic man, who fulfills the requirements to become a bishop or priest, can become pope," said Alfred Hierold, a professor for church law at Bamberg University.

While the "servant of the servants of God" could therefore be a layman, he would quickly have to be made bishop as the pope is also always the bishop of Rome. That problem hasn't arisen for centuries, however: Since 1378, every pope has been a cardinal before his election.

Gathering in Rome, cardinals will stay at "Domus Sanctae Marthae," a spartan guest house behind St. Peter's Cathedral in Vatican City.

"The rooms have a metal bed, a large desk, a chair, a small bathroom and stone floors," said Hierold, who has stayed there himself.

Complete reclusion

On the first day of the conclave, cardinals celebrate a special mass called "pro eligendo papa," or "for the election of the pope." Traditionally, they then retreat to the Vatican's Sistine Chapel to begin their deliberations. However, this time the cardinals will not be confined to the chapel. They'll be permitted to roam about within the walls of the Vatican, though they will still be banned from contact with the outside world.

"If a cardinal brings along his mobile phone, there's nothing that can be done," Hierold said, adding that the ballot box is placed underneath Michelangelo's famous "Last Judgement" fresco. "But it's not allowed."

The chapel is even checked for listening devices and cardinals as well any support staff have to swear that they will remain silent about anything that takes place within and that they will not be influenced by others in their decision.

Bribes and intrigues

There's good reason for that as bribing and intrigues were not uncommon at previous papal elections. Until 1903, European monarchs also played a role in electing the leader of the Catholic Church.

The cardinals keep meeting until they have agreed on a new pope. The longest conclave lasted three years. In order to be elected pope, a candidate has to receive two thirds of the votes. Should he not receive them right away, four rounds of votes are cast each day, separated by prayers. The first break will take place if a new pontiff has not been elected after three days.

Should cardinals fail to agree after 34 voting rounds, they can decide to elect a pope by majority.

Dark smoke, white smoke

Smoke is used to announce the result of each vote as the ballots are burned each time. If the vote has been unsuccessful, wet straw is added to the fire, causing dark smoke to rise above the chapel. Once a pope has been chosen, dry straw is added and white smoke signals the beginning of a new pontificate. I

In addition to white smoke, the faithful will this time also hear the ringing of the bells of St Peter's Cathedral to announce a decision.

Inside, cardinals acknowledge their new supreme leader as well. Everyone except the new pontiff folds up the little canopy above his seat. The new pope is then asked what name he chooses and officially becomes pope once a document has been drawn up that certifies his election.

John Paul II's successor will then put on the white papal clothes and accept a pledge of allegiance by the other cardinals. Only then will the dean of the college of cardinals, Germany's Joseph Ratzinger, step on the balcony of St. Peter's cathedral.

"Annuntio vobis gaudium magnum, habemus papam," he'll tell the cheering crowd. "I announce to you with great happiness: We have a pope."

This time, the latter might get a bit complicated, as Ratzinger himself has good chances of becoming the next pope.