Vatican City, March 11 - According to tradition, the process of electing a new pope will begin Tuesday morning with a special mass, 'Pro Eligendo Papa', in St Peter's Basilica. In the afternoon, the cardinals who have gathered from all over the world will assemble in the basilica's 'Aula della Benedizione' to invoke the guidance of the Holy Spirit by singing the Latin hymn Veni Creator (Come Holy Spirit). They then proceed to the Sistine Chapel, where they begin the process of choosing the pope's successor. This highly secretive meeting under Michelangelo's famous frescoed ceiling is known as the conclave, from the Latin cum clave, which means literally 'with-key', because in the past cardinals were often locked into a room while the voting was going on. Nowadays they are not going to be locked into the Sistine Chapel but, in the interests of secrecy, they will be confined to the Vatican and contact with the outside world will be cut to a minimum. Only cardinals, the 'princes' of the Church, can vote for popes. Paul VI excluded cardinals over 80 from the election and limited electors to 120. More than half, 67, of the cardinal-electors were appointed by Benedict and the remaining 50 by his predecessor John Paul II. The 117 are now down to 115 after Indonesia's retired Archbishop of Jakarta, Julius Darmaatjadja, pulled out because of illness and Britain's top official, Cardinal Keith O'Brien of Scotland, resigned over allegations of "inappropriate" behaviour with priests. A majority of the cardinals, 59, are European, and 28 Italian, raising the chances of the first Italian pope since the short-lived John Paul I, who ruled from 26 August 1978 until his death 33 days later. In 2007 Benedict tweaked the rules so that a two-thirds majority instead of simple majority - used to break deadlocks in protracted voting - would always be required for electing a new pope. That magic number is 77. The previous major legislation on conclaves was in the apostolic constitution 'Universi Dominici Gregis' (The Lord's Whole Flock), published by Pope John Paul in 1996. It confirmed the limit of 120 cardinals. 'Universi Dominici Gregis' forbids electioneering and deal-making. It says: "The cardinal electors shall abstain from any form of pact, agreement, promise or other commitment of any kind." "(They are) not to allow themselves to be guided...by friendship or aversion, or to be influenced by favor or personal relationships towards anyone, or to be constrained by the interference of persons in authority or by pressure groups, by the suggestions of the mass media, or by force, fear or the pursuit of popularity". The so-called sin of simony or voting deals carry the possible punishment of excommunication. Each elector writes the name of his chosen candidate on his ballot paper under the words Eligo in Summum Pontificem (I elect as supreme pontiff). He then walks to the altar and slides his slip in the ballot box saying: "I call as my witness Christ the Lord who will be my judge that my vote is given to the one whom I think should be elected." There are four ballots per day and, at least to begin with, a majority of two thirds of the votes is required. If no one has been chosen after three days, there is a pause of up to one day for prayer, "informal discussion among the voters," and a brief spiritual exhortation by the senior cardinal-deacon. At this point, the so-called "great electors" - cardinals good at negotiating agreements and organizing voting blocs - come into their own. After the pause, voting continues for another seven ballots, followed by another pause, seven more ballots, and so on, until there have been up to 30 ballots extending over 10 or 12 days. John Paul introduced the simple majority as a way of trying to break a possible stalemate at this point, but Benedict, an even bigger traditionalist than his predecessor, changed the bar back to 66% because he felt any pope needed as much legitimation as possible, for a stronger mandate. As the name conclave suggests, secrecy is a paramount consideration throughout the process and the last two popes came up with several new rules to preserve it. Security specialists will check the Sistine for bugs. The cardinals are not to communicate with anyone outside. Nor are they allowed access to phones, both mobile and landlines, newspapers, radio or TV. Despite the Vatican's recent opening to social media, including Facebook and Twitter, the Internet is also strictly off limits.