Pope Francis’s successor will be chosen in an arcane, secretive process known as a conclave, in which cardinals from around the world gather beneath the frescoed ceilings of the Sistine Chapel in the heart of the Vatican.

The conclave is a centuries-old tradition that mixes theatre, theology and the utmost secrecy.

For a few days it will restore to Rome its ancient title – “caput mundi” or capital of the world – as 1.2 billion Catholics await the result of the election.

All cardinals under the age of 80 are eligible to vote in the conclave, a word which derives from the Latin “cum clave”, with a key, because cardinals used to be shut behind locked doors until they came to a decision.

Once the cardinals arrive in Rome from around the world, they hold a special mass in St Peter’s Basilica called “Pro Eligendo Romano Pontifice” (”For the Election of the Roman Pontiff”).

They then walk – or in the case of the more elderly and infirm, take a mini-bus – to the Apostolic Palace.

Dressed in red robes over white lace tunics, from there they will process to the Sistine Chapel, led by a choir singing the Litany of Saints, a Gregorian chant imploring the intercession of the Holy Spirit to help guide voting.

The chapel, which is decorated by frescoes by Michelangelo and other Renaissance artists, takes its name from Pope Sixtus IV, who had it built between 1477 and 1480.

One by one, they will approach the altar beneath Michelangelo’s Last Judgment, place their hands on a Bible and take an oath of secrecy in Latin by which they “promise and swear to observe with the greatest fidelity and with all persons, secrecy regarding everything that in any way relates to the election of the Roman Pontiff.”

The Last Judgment luridly portrays tortured souls being consigned to eternal damnation by wild-eyed demons.

Oath of secrecy 

The chapel will have been swept for bugs and other recording devices and cardinals will be banned from using laptops or mobile phones.

For the last conclave in 2013, which followed the historic resignation of Pope Benedict XVI, an oath of secrecy was also taken by members of the Swiss Guard and Vatican gendarmerie, the city state’s police force, as well as the doctors and nurses who assisted elderly or infirm cardinals.

A Vatican master of ceremonies will pronounce the words “Extra omnes” (“Everyone out”), ordering staff and aides to leave the chapel so that only the cardinal electors remain.

The late Cormac Murphy-O’Connor, a British cardinal and the then Archbishop Emeritus of Westminster, gave an insight into what it was like to take part in a conclave.

“It’s an extraordinary moment. You all process in. Then one of the cardinals says ‘omnes extra”,” he told the BBC.

“The preacher and others all went out and I heard the thud of the door of the Sistine Chapel. I remember looking around at all of the other 114 cardinals and thinking: ‘One of us will be going out with a white cassock on.’”

The cardinals – known as “princes of the Church” – will be seated on chairs made of cherry wood behind 12 wooden tables covered in satin and cloth.

There will be a 13th table positioned in front of the altar, on which will be placed a silver urn into which the cardinals will cast their secret ballot papers.

Selection process 

Each cardinal writes his choice on a ballot paper inscribed with the words “Eligo in summum pontificem,” or “I elect as Supreme Pontiff.”

One ballot is held on the first day of the papal conclave.

For the rest of the conclave they will vote four times a day – twice in the morning and twice in the afternoon - burning their ballot papers after every second vote, until they reach a two-thirds majority.

Two metal stoves will have been installed inside the 15th-century chapel, connected by a long metal pipe to the chimney on the roof.

One stove will be used for the burning of ballot papers after cardinals cast their votes, while the other will be used to emit coloured smoke - black to signify that the cardinals have yet to reach agreement, white to announce “habemus papam” - we have a Pope.

Chemicals are used to create the different coloured smoke, although to avoid ambiguity, the white smoke that signifies a pope has been elected will be accompanied by the ringing of St Peter’s bells.

Once the two-thirds majority has been achieved, the most senior cardinal will ask the chosen cardinal if he is prepared to become Pope.

Deciding the name

If the answer in Latin is “Accepto” (I Accept), he is taken into a side room known as the Stanza delle Lacrime, or Room of Tears, so named because many have wept at the enormity of the task they face.

There he is given help to take off his scarlet cassock and change into the white vestments of the papacy.

He also has to decide by what name he wishes to be called – the name-changing tradition dates back to the sixth century when a priest named Mercury who was elected Pope felt his name was too pagan and changed it to John II.

The senior cardinal will then step out onto the balcony of St Peter’s Basilica and announce in Latin: “Annuntio vobis gaudium magnum. Habemus papam!” (I announce to you a great joy. We have a pope!).

He will announce the name of the elected cardinal and the papal title he has chosen.

The new Pope then gives his first blessing, watched by huge crowds in St Peter’s Square and on television by millions of people around the world.

The ceremony to install the new Pope takes place a few days later.