Preparations for the election of the pope began: Conclaves lasted from a few hours to three years

Yet in the 13th century, it took nearly three years, 1,006 days to be exact, to elect a successor to Pope Clement IV, making it the longest conclave in the history of the Catholic Church.

This is where the term conclave comes from - "under lock and key", because the cardinals meeting in Viterbo, north of Rome, needed so long that the locals decided to lock them in a room until they chose a new pope.

The secret ballot that elected Pope Gregory X lasted from November 1268 to September 1271, and was the first example of a papal election by "compromise", after a long struggle between supporters of the two main geopolitical medieval factions - those loyal to the papacy and those supporting the Holy Roman Empire.

Gregory X was elected only after the people of Viterbo removed the roof from the building where the prelates stayed and limited their meals to bread and water in order to pressure them into making a decision, and hoping to avoid this happening again, Gregory X decreed in 1274 that the cardinals would receive only "one meal a day" if the conclave lasted more than three days, and only "bread, water and wine" if it lasts longer than eight, which was later abolished.

Before 1274, there were cases when a pope was elected on the same day his predecessor died, but after that the church decided to wait at least 10 days before the first vote, which was extended to 15 to allow all the cardinals to arrive in Rome.

The fastest conclave to observe the 10-day waiting rule was the election of Pope Julius II in 1503, who was elected in just a few hours, according to Vatican historian Ambrogio Piazoni.

AP Photo/Andrew Medichini

More recently, Pope Francis was elected in 2013 on the fifth ballot, Benedict XVI won in 2005 on the fourth, and Pope Pius XII on the third in 1939.

The first conclave held in the Sistine Chapel was held in 1492, and in 1878, the famous chapel became the venue for all conclaves.

"Everything contributes to the awareness of the presence of God, before whose eyes every person will one day be judged," wrote Pope John Paul II in the 1996 document, "Universi Dominici Gregis," which regulates the conclave.

Most of the conclaves were held in Rome, and some took place outside the walls of the Vatican. Four were held in the Pauline Chapel of the papal residence in the Quirinal Palace, while around 30 others were held in the Basilica of St. John Lateran, Santa Maria Sopra Minerva or other places in Rome.

On 15 occasions conclaves were held outside Rome and the Vatican, including Viterbo, Perugia, Arezzo and Venice in Italy, and Constance in Germany and Lyon in France.

Between 1378 and 1417, in a period historians call the Western Schism, there were rival claimants to the title of pope, and the election of a pope and his rival, the so-called "antipope," divided the Catholic Church for nearly 40 years.

The schism was finally resolved by the Council of Constance in 1417, which led to the election of Martin V, the universally accepted pope, reports AP.

The closure of the conclave presented another challenge for the cardinals: maintaining health and hygiene, and before the Santa Marta home was built in 1996, where the cardinals now stay during the conclave, they slept on beds in the rooms connected to the Sistine Chapel.

Conclaves in the 16th and 17th centuries were described as "disgusting" and "bad-smelling", with concerns about disease outbreaks, especially in summer, according to historian Miles Pattenden.

Tanjug/Vatican Media via AP

"The cardinals simply had to have a more comfortable lifestyle because they were older people, many of them with fairly advanced disease," Pattenden wrote.

The confined space and lack of ventilation further exacerbated these problems, and some of the cardinal electors left the conclave ill, often seriously.

At first, papal elections were not so secretive, but concerns about political interference grew during the longest conclave in Viterbo.

Gregory X decreed that the cardinal electors should be locked in isolation, "cum clave" (under lock and key), until a new pope was elected, the goal being to create a completely isolated environment where the cardinals could focus on the task at hand, guided by God's will, without any political interference or distraction.

The youngest pope in history was John XII, who was only 18 years old when he was elected in 955.

On the other hand, the oldest popes were Pope Celestine III (elected in 1191) and Celestine V (elected in 1294), both of whom were almost 85 years old.

Any man who has been baptized in the Roman Catholic Church and is over 18 years old can become the Pope, but for more than 600 years it has been exclusively cardinals.

The last time a non-cardinal pope was elected was Urban VI in 1378, who was a monk and archbishop Baria.

Italians were elected pope the most times, but apart from the last three popes, the Spaniard Alexander VI Borgia, who was Spanish, the Syrian Gregory III and the Dutch Hadrian VI were among the foreigners.

Pope Francis was an Argentine of Italian origin, Benedict XVI was German, and his predecessor John Paul II was from Poland.