For centuries, leaders of the Roman Catholic Church have been chosen at the Vatican in private gatherings known as conclaves.
Much secrecy surrounds this conclave and its historic vote, which usually happens in the days after a pope dies. But this year brings a rare twist: For the first time in 600 years, a pope has resigned.
Technically, any Roman Catholic male can be elected pope. But since 1379, every pope has been selected from the College of Cardinals, the group casting the votes at the conclave.
Many of the cardinals are bishops and archbishops appointed by the pope to assist in religious issues. Some work at the Vatican, but most are spread out worldwide running dioceses or archdioceses.
When it's time to vote for a new pope, every cardinal under the age of 80 travels to Rome to participate. In attendance this time will be 115 cardinals, 67 of whom were appointed by Pope Benedict XVI, who stepped down last month at age 85.
Roll over the squares below to see the names of the cardinal electors and where they're from, broken down by region.
Once all the cardinals have arrived, the conclave begins with a special morning Mass in St. Peter's Basilica. In the afternoon, the cardinals walk to the Sistine Chapel -- with its iconic Michelangelo frescoes -- to start the voting process.
The vote is held behind closed doors, and its secrecy is closely guarded. The chapel is checked for hidden microphones and cameras, and the cardinals are not allowed to talk about the proceedings with anyone outside the group. If they do, they could be excommunicated.
Inside the Sistine Chapel, paper ballots are passed out to each cardinal, who writes the name of their chosen candidate below the words "Eligo in Summun Pontificem" (Latin for "I elect as supreme pontiff"). Cardinals cannot vote for themselves.
See possible papal contenders »
When they're done, each cardinal -- in order of seniority -- walks to an altar to ceremoniously place his folded ballot into a chalice. The votes are then counted up and the result is read to the cardinals.
If a cardinal has received two-thirds of the vote, he becomes the new pope.
If there is no pope, as many as four votes a day -- two in the morning and two in the afternoon -- can be held on the second, third and fourth days of the conclave. The fifth day is set aside to break for prayer and discussion, and then voting can continue for an additional seven rounds. After that, there's another break and the pattern resumes.
We can't get into the Sistine Chapel, but we'll know whether there's a new pope simply by watching the smoke that comes from the Vatican's rooftop.
Ballots are burned after the votes, once in the morning and once in the afternoon. If a pope hasn't been chosen, the ballots will be burned along with a chemical that makes the smoke black.
If the smoke is white, however, the world's 1.2 billion Roman Catholics have a new head of the church.
Traditionally, about 30 to 60 minutes after the white smoke, the new pope will appear on the balcony overlooking St. Peter's Square. Cardinal Jean-Louis Tauran, if he's not elected pope himself, will announce the words "Habemus Papam" (Latin for "We have a pope") and introduce the new pope by his chosen papal name.
The new pope will then speak briefly and say a prayer. His formal coronation will take place days after his election. The last two popes have been inaugurated in St. Peter's Cathedral.
Produced by Kyle Almond and Sean O'Key. Design by Sophia Dengo. Animation by Rob Wright and John Cowan. Photos from Getty Images.