1268–1271 papal election
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The papal election that followed the death of Pope Clement IV lasted from 1 December 1268 to 1 September 1271 and was the longest in the history of the Catholic Church. This was due primarily to political infighting between the cardinals. The election of Teobaldo Visconti as Pope Gregory X was the first example of a papal election by "compromise", that is, by the appointment of a committee of six cardinals agreed to by the other ten. (This method was attempted once before, in the 1227 papal election, but the choice of the committee refused the honor and the full group of cardinals proceeded to elect the pope.) The election occurred more than a year after the magistrates of Viterbo locked the cardinals in, reduced their rations to bread and water, and removed the roof of the Palace of the Popes in Viterbo where the election took place. As a result of the length of the election, during which three of the twenty cardinal-electors died and one resigned, Gregory X promulgated the papal bull Ubi periculum on 7 July 1274, during the Second Council of Lyon, establishing the papal conclave, whose rules were based on the tactics employed against the cardinals in Viterbo. The first election held under those rules is sometimes viewed as the first conclave.