Cardinals attend a mass in St. Peter's Basilica at the Vatican March 12, 2013. All cardinals, including those over 80 who will not vote in the conclave, celebrate Mass in St Peter's Basilica to pray for the election of the new pope. The Mass is called "Pro Eligendo Romano Pontefice" ("For the Election of the Roman Pontiff") and is open to the public
Roman Catholic cardinals begin a conclave on Tuesday to elect the Church's 266th pontiff and a successor to Pope Benedict, who abdicated unexpectedly last month.
Following are key facts about the conclave - one of the world's oldest and most secret electoral processes.
WHO - 115 cardinals who were aged under 80 when Benedict abdicated are participating. Two other eligible cardinals are not attending - one for health reasons, one because of his involvement in a sex scandal. Ninety cardinals aged 80 or over cannot take part. The person chosen as pope does not have to be one of the cardinal electors, but in practice now always is.
The elector cardinals come from 48 countries. Italians make up the biggest single national bloc, with 28 cardinals against 11 from the United States, six from Germany and five each from India and Brazil. Sixty cardinals come from Europe, 19 from Latin America, 14 from North America, 11 from Africa, 10 from Asia and one from Oceania.
WHERE - The cardinals start their meeting at 4:30 p.m. (11.30 aa.m. ET) in the Sistine Chapel, under Michelangelo's frescoes of the Last Judgment and of Bible scenes including the creation panel with the finger of God and the finger of Adam nearly touching. Cardinals sleep in a Vatican hotel behind St Peter's Basilica. They will be banned from communicating with the outside world - no phones, television or Internet.
HISTORY - 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. One conclave in the 13th century lasted two years, nine months and two days. The average length of the past nine conclaves was just over three days. The last conclave, which elected Benedict in 2005, lasted barely 24 hours.
BALLOTING - The cardinals will almost certainly cast their first vote on Tuesday. From Wednesday they will vote twice each morning and twice each afternoon. The cardinals will suspend voting on Saturday if they have not chosen a pontiff and resume their balloting on Sunday. To win, a candidate needs a two-thirds majority, or at least 77 votes.
SMOKE - After cardinals cast their votes on papers printed with the Latin words "Eligo in Summum Pontificem" ("I choose as Supreme Pontiff"), the ballots are burned and smoke pours from a makeshift chimney above the Sistine Chapel.
The smoke signals, telling the world whether or not cardinals have elected a new pope, are expected at around noon (7.00 a.m. ET) and 7 p.m. (2.00 p.m. ET) each voting day. However, smoke could emerge earlier if the new pontiff is elected in the first ballot of one of the sessions.
Black smoke marks an inconclusive vote; white smoke and the tolling of the bells of St. Peter's Basilica mean a pope has been elected.
"HABEMUS PAPAM" - 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"). He identifies the new pope by his given name, with his first name translated into its Latin version, and then announces the papal name the new leader of the Church has chosen.
The papal names most often chosen have been John (23 times), Gregory (16), Benedict (16), Clement (14), Innocent (13), Leo (13) and Pius (12).
After the announcement, the new pope steps forward to deliver his first public address and his first "Urbi et Orbi" ("To the City and the World") blessing in front of the crowds gathered in St. Peter's Square.