It's the oldest enduring electoral system in the world and many of its traditions have been unchanged for centuries.
The conclave, which literally means "locked with a key", dates back to a time when cardinals 'were' locked-in until they chose a new pope.
Now, it's the world that is locked out, figuratively speaking, as much of the conclave will take place behind closed doors.
The gathering begins with a morning mass in St. Peter's Basilica.
In the afternoon, the 115 voting cardinals of the Roman Catholic church, those under 80 years old, enter the Sistine Chapel where each will take an oath of secrecy.
The penalty: automatic excommunication.
After the oath -- preparations are made for the election -- taken by secret ballot.
Lots are drawn to select three cardinals who will help collect ballots: three more cardinals to count the votes and three others to review the results.
Printed on the ballots -- the words: "eligo in summum pontificem," -- meaning "I elect as supreme pontiff."
Each elector writes the name of one candidate on the lower half of the ballot -- and folds it in half.
Cardinals are not allowed to vote for themselves.
Then, in order of seniority -- the cardinals take their ballots to the altar.
Each places a folded ballot on to a small disc and then the ballot is dropped into a chalice.
Once all the votes are cast, the ballots are tallied and the results are read aloud.
More than a two-thirds majority is needed to declare a winner, in this case 77-votes.
If there is no winner, there's another vote.
If there is still no winner, two more votes are scheduled for the afternoon.
Voting continues, up to four ballots each day until there's a winner.
The ballots are burned after each session in an incinerator inside the chapel, sending off the most famous smoke signal in the world.
If there's no winner, they're burned with a chemical that gives off black smoke, telling the crowd waiting in St. Peter's Square that a new pope has not yet been selected.
When there is a winner the ballots are burned alone, giving off white smoke - a sign from the cardinals that they have chosen a new pope to lead the church.
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