What was the longest papal conclave, and how did Viterbo invent the lock-in?

The longest papal election in the history of the Catholic Church began on 1 December 1268, after the death of Pope Clement IV, and ended on 1 September 1271, almost three years later. It took place in the Italian town of Viterbo and dragged on primarily because of political infighting between the cardinals. It is also where the idea of locking cardinals in came from: the rules of the conclave, issued by Pope Gregory X in 1274, were based on the tactics used against the cardinals in Viterbo.

The deadlock had a clear shape. The French Angevin cardinals, mostly created by Pope Urban IV, were amenable to an invasion of Italy by Charles of Anjou. The non-French cardinals, mostly Italian, had just enough numbers to prevent a French pope from being elected. Of the twenty cardinal-electors, three died during the election and one resigned.

In late 1269, after months of deadlock, the town's officials ordered the cardinals sequestered in the Palace of the Popes until a pope was elected. The magistrates later reduced their rations to bread and water and removed the roof of the palace. According to one account, by Onofrio Panvinio, Cardinal John of Toledo suggested it himself: "Let us uncover the Room, else the Holy Ghost will never get at us". Other sources say Charles of Anjou orchestrated the hardships, and some say a makeshift roof was put back after the cardinals threatened to put all of Viterbo under interdict.

In the end the cardinals ceded their authority to a committee of six, which chose Teobaldo Visconti, who was not even a cardinal and was in Acre as papal legate to a crusade. He took the name Gregory X, and in 1274 his bull Ubi periculum set the rules: seclusion for the whole election, meals passed through a small opening, one meal a day after three days, and bread and water after eight.