Gregory X owed his own election to a crisis the council was determined not to repeat. After Pope Clement IV died in 1268, the cardinals gathered to choose a successor took nearly three years to agree, a deadlock so prolonged that the citizens of Viterbo, where the cardinals were meeting, are said to have locked them into the episcopal palace and reduced their food to bread and water in an effort to force a decision, the origin of the word conclave, from the Latin for a room that can be locked with a key.
The Second Council of Lyons wrote a version of that improvised pressure into permanent church law: cardinals were now required to be sequestered together within ten days of a pope's death, denied contact with the outside world, and given progressively reduced rations the longer the election dragged on, rules intended to make a repeat of the Viterbo vacancy structurally difficult rather than merely embarrassing. The union with Constantinople proclaimed at the same council did not survive Michael VIII's death, but the conclave procedure did, and remains, with later modification, the basis of how the Catholic Church elects a pope today. Later councils tightened the procedure further, but the core principle Lyons established, that only the cardinals decide and that a supermajority is required, has governed every conclave since.