The idea of cardinals picking a new pope dates back centuries — but the conclaves Catholics have come to know today first began in the 13th Century,  after the longest conclave ever elected Pope Gregory X.

It took two years and nine months to do so, and it was then that holy father who would create the rules the red hats still follow today.

Technically, any Roman Catholic male can be elected the leader of the universal Church, but since the 14th Century, every pope has previously been a cardinal.

The task of electing the next successor of St. Peter falls to the College of Cardinals.

They must be younger than 80 years old to participate, and all eligible candidates must vote in person.

Before they start voting all the red hats, including those over 80, gather for crucial meetings in Rome.

It is there that the big questions facing the Church are discussed and potential candidates start to emerge.

The conclave officially begins with a prayer service in the Pauline Chapel, located in the Apostolic Palace in Vatican City

The princes of the Catholic Church will pray the Litany of the Saints before entering in procession into the Sistine Chapel, where they are sealed inside.

They aren’t allowed to speak to anyone outside of the process, read media reports, receive messages, or discuss the details of the elections afterwards.

If they break the conclave secrecy rules — they risk excommunication.

Once the voting starts they write a candidate’s name on a ballot and in order of seniority, they drop them into a chalice.

Cardinals aren’t allowed to vote for themselves.

The results are recorded by three designated cardinals.

To be elected pope, a candidate needs to get two-thirds of the vote.

They’ll cast their votes four times a day, every day until they reach that threshold and the man chosen accepts the decision of the cardinals.

If no man is elected after 4 days the conclave breaks for prayer and discussion before trying again.

The ballots are burned after each round of voting.

If there’s no winner, a chemical is added to turn the smoke black, so the people waiting in Saint Peter’s Square know that a new pope has not yet been chosen.

But when the pope has been elected, a different chemical is added to ensure the smoke comes out white — signaling a new pontiff.

Finally, a senior cardinal tells the world that we have a pope and introduces the new Holy Father to the crowd gathered in Saint Peter’s Square.

Then the new pope goes out onto the balcony to make his first remarks and give his blessing.

Since 1900, the average conclave has lasted about three days.

Pope Francis and Pope Benedict the Sixteenth both took two days to be elected.