The 115 cardinals gathering today to choose a new pope must be hoping to finish the job in less than the two years, nine months and two days it took when Pope Nicholas IV died in 1292.

Members of the conclave began their task of choosing the 265th pontiff by celebrating Mass in public at St Peter's Basilica in the Vatican at 10am (9am UK time).

All the electors are under the age of 80, according to strict rules governing what is probably the world's oldest and most secretive ballot. Two cardinals are ill, and cannot attend, while a further 66 are disqualified from voting because they are too old.

At 4.30pm (3.30pm UK time) they will retreat from public sight, processing into the Sistine Chapel to take their oath of secrecy and hear a meditation from a senior cardinal. Seated underneath Michaelangelo's famous frescoes of Bible scenes, they will then decide whether to take a first vote today or wait until tomorrow morning.

Overnight they return to their comfortable en-suite bedrooms in the luxurious new Vatican hotel, which for the first time will replace the cramped cells around the Sistine Chapel where the conclave has traditionally been housed. They will be banned from communicating with the outside world - no phones, newspapers, television, radio or internet.

Tomorrow their routine starts in earnest, with Mass at 7.30am in the hotel chapel. At 9am the cardinals will gather in the Sistine Chapel for their initial two rounds of voting, inscribing their choice on papers printed with the Latin words Eligo in Summum Pontificem (I choose as Supreme Pontiff).

The first puff of smoke from the makeshift chimney above the Sistine Chapel is expected at noon. If the smoke is white and the bells of St Peter's start to toll, it will indicate that a new pope has already been chosen. If, as is more likely, the smoke is black, then it will indicate that no decision has been made in the morning session of voting. The smoke is created by burning the ballot papers, with chemicals additives to change the colour.

At 4pm the cardinals will return to the Sistine Chapel for the two rounds of afternoon balloting, with the result announced to the world at 7pm with another smoke signal.

The rest of the days of the conclave are expected to follow Tuesday’s schedule, with two votes in the morning and two in the afternoon. A Vatican spokesman said smoke signals from burned ballot papers could likely be seen at around noon and 7pm each day, unless a winner is elected today in a first ballot, in which case it could emerge earlier. After every three days, the cardinals suspend voting for a day.

The word conclave (from Latin "cum clave", or "with a key") dates back to the protracted election of Celestine IV in 1241 when cardinals were locked up in a crumbling palace.

The duration of the vote varies enormously, although the average length of the eight conclaves in the 20th century was just over three days. The last conclave which elected John Paul II in 1978 lasted fewer than three days.

The person chosen as pope does not necessarily have to be one of the cardinal electors, but in practice almost always is.

Cardinal Joseph Ratzinger of Germany, a 78-year-old with conservative views, is the favourite. To win, a candidate needs a two-thirds majority, or at least 77 votes. After 33 or 34 ballots, cardinals can decide to switch to a majority vote.

When a pope is chosen, a senior cardinal appears on the balcony of St Peter’s Basilica and announces in Latin: Annuntio vobis gaudium magnum. Habemus Papam (I announce to you great joy. We have a pope).

The new pope later steps forward to deliver his first public address and his first Urbi et Orbi (To the City and the World) blessing to the crowds gathered in St Peter’s Square. Once the new pope has accepted, he picks a name. Those most often chosen have been John (23 times), Gregory (16), Benedict (15) Clement (14), Innocent (13) and Leo (13).

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