0n 1 September 1271, Pope Gregory X was elected Pope at the end of what would be the longest papal election in history. It will also be the first example of a Papal election by compromise and lead Gregory X to issue reforms in the selection process of new popes to ensure that such a long delay in the election of a new pontiff was not repeated.

Cardinals began the process of electing a successor to Pope Clement IV in November 1268 but were divided between French and Italian cardinals – each of whom hoped to elect one of their number. Once each day the cardinals would meet at the Viterbo Cathedral to vote. After months of deadlock, the Podesta of Viterbo ordered the cardinals to be sequestered until a new pope was elected. The diet of the cardinals was reduced to bread and water and the roof of the papal palace was removed, all to encourage the cardinals to more quickly choose a new pope. Under pressure from King Philip III, the cardinals would eventually form a committee of six who would agree to elect Tebaldo Visconti as Clement IV’s successor.

Clement X in his Apostolic Constitution Ubi Periculum required the sequestration of cardinals for future papal elections and a limit on the food provided the cardinals if they did not choose a new pope within three days. Pope John XXI would later revoke Ubi Periculum.