The voting by cardinals to elect the next pope takes place behind the locked doors of the Sistine Chapel, following a highly detailed procedure last revised by the late Pope John Paul II.

Under the rules, secret ballots can be cast once on the first day of the conclave, then normally twice during each subsequent morning and evening session. Except for periodic pauses, the voting continues until a new pontiff is elected.

Only cardinals under the age of 80 can vote in the conclave; older cardinals do not enter the Sistine Chapel. In theory, any baptized male Catholic can be elected pope, but current church law says he must become a bishop before taking office; since the 15th century, the electors always have chosen a fellow cardinal.

Each vote begins with the preparation and distribution of paper ballots by two masters of ceremonies, who are among a handful of non-cardinals allowed into the chapel at the start of the session.

Then the names of nine voting cardinals are chosen at random: three to serve as “scrutineers,” or voting judges; three to collect the votes of any sick cardinals who remain in their quarters at the Domus Sanctae Marthae; and three “revisers” who check the work of the scrutineers.

The paper ballot is rectangular. On the top half is printed the Latin phrase “Eligo in Summum Pontificem” (“I elect as the most high pontiff”), and the lower half is blank for the writing of the name of the person chosen.

After all the non-cardinals have left the chapel, the cardinals fill out their ballots secretly, legibly and fold them twice. Meanwhile, any ballots from sick cardinals are collected and brought back to the chapel.

Each cardinal then walks to the altar, holding up his folded ballot so it can be seen, and says aloud: “I call as my witness Christ the Lord who will be my judge, that my vote is given to the one who before God I think should be elected.” He places his ballot on a plate, or paten, and then slides it into a receptacle, traditionally a large chalice.

When all the ballots have been cast, the first scrutineer shakes the receptacle to mix them. He then transfers the ballots to a new urn, counting them to make sure they correspond to the number of electors.

The ballots are read out. Each of the three scrutineers examines each ballot one-by-one, with the last scrutineer calling out the name on the ballot, so all the cardinals can record the tally. The last scrutineer pierces each ballot with a needle through the word “Eligo” and places it on a thread, so they can be secured.

After the names have been read out, the votes are counted to see if someone has obtained a two-thirds majority needed for election—or a simple majority if the rules are changed later in the conclave. The revisers then double-check the work of the scrutineers for possible mistakes.

At this point, any handwritten notes made by the cardinals during the vote are collected for burning with the ballots. If the first vote of the morning or evening session is inconclusive, a second vote normally follows immediately, and the ballots from both votes are burned together at the end.

When a pope is elected, the ballots are burned immediately. By tradition, the ballots are burned dry—or with chemical additives—to produce white smoke when a pope has been elected; they are burned with damp straw or other chemicals to produce black smoke when the voting has been inconclusive.

The most notable change introduced by Pope John Paul II into the voting process was to increase the opportunity of electing a pope by simple majority instead of two-thirds majority, after a series of ballots. The two-thirds majority rule holds in the first phase of the conclave: three days of voting, then a pause of up to one day, followed by seven ballots and a pause, then seven more ballots and a pause, and seven more ballots.

At that point—about 12 or 13 days into the conclave—the cardinals can decide to move to a simple majority for papal election and can limit the voting to the top two vote-getters. In earlier conclaves, switching to a simple majority required approval of two-thirds of the cardinals, but now that decision can be made by simple majority, too.