London: When Pope Benedict resigns on 28 February he will leave the office vacant and the process to choose a new leader of the world’s 1.2 billion Catholics will begin.

Without the customary mourning period that follows the death of a pontiff, a meeting of the cardinals to elect his successor, called a conclave, could begin by mid-March.

The word comes from the Latin cum clave (“with key”) and refers to the fact that the Roman Catholic church’s most senior prelates used to be locked in the Sistine Chapel and adjoining buildings. The idea was to make them as uncomfortable as possible to force a choice.

It first took hold in Viterbo, a town in central Italy that was the site of several papal elections in the middle ages at times when Rome itself was judged to be too turbulent.

In 1271 the cardinals had spent 33 months failing to make up their minds, largely for political reasons, when the people of Viterbo lost patience. They persuaded the local authorities to lock the cardinals in a fortress, cut their food rations and remove the roof of the fortress to expose them to the elements.

The cardinals soon chose Gregory X, who, three years later, introduced new rules for the election of popes, including one that said the cardinals had to meet in seclusion on a gradually reduced diet until they were living off bread and water. His successor was elected in a day; the next pope in seven.

Until the election of Benedict in 2005, a conclave was something to be approached with as much dread as reverence. The mainly elderly cardinals were lodged in a walled-off space within the Apostolic Palace, divided into small, bare rooms.

Since the introduction of rules approved in 1996 by his predecessor, John Paul II, the cardinal-electors have been given comfortable accommodation in a new guesthouse within the Vatican, a sizeable complex of 108 suites and 23 single rooms, all with private bathrooms. The prelates will travel by bus to and from the Sistine Chapel, where the voting takes place beneath Michelangelo’s fresco of the Last Judgment.

The conclave that chose Benedict began on 18 April 2005 and ended the following day after four ballots. But not all cardinals take part in a conclave. Anyone who has been nominated in pectore – anonymously, so as to protect them from reprisals or for some other reason – is excluded. So are the many cardinals who are over 80.

By the time the conclave begins, there is likely to be at least some consensus over the leading contenders. But Catholics believe that, once behind the shut doors of the Sistine Chapel, the cardinals are guided to their decision by God directly in the guise of the Holy Spirit. Anything becomes possible.

Certainly, there have been some remarkable shocks in the 2,000 years of the papacy, few more unexpected than the election of Karol Wojtyla as John Paul II in 1978. Joseph Ratzinger, however, went into conclave a favourite from which he emerged as Benedict XVI.

The cardinal-electors are forbidden to exchange messages of any kind with the outside world for the duration of the conclave. A dominant concern of John Paul’s 1996 edict, Universi Dominici Gregis (The Shepherd of the Lord’s Whole Flock) was to ensure deliberations remained secret, even after they had reached a decision. Anyone in Vatican City who should happen to meet one of the cardinal-electors during the election is forbidden to engage in conversation of any sort with the cardinal.

Universi Dominici Gregis also stipulates “careful and stringent checks” must be made to ensure no audio-visual equipment has been secretly installed in or around the Sistine Chapel “for recording and transmission to the outside”.

Ballot slips are customarily burnt in a stove whose chimney extended through a window of the Sistine Chapel. When there was no result, straw was mixed with the ballots to produce thick, black smoke as a signal to those waiting outside. Sometimes, the difference between black and white smoke has been difficult to discern.

In 1958, when John XXIII was elected, Vatican Radio’s reporters got it wrong and told the world a pope had been chosen a day before the decision was reached.

—Guardian News & Media Ltd