The death of Pope Francis has triggered a period of mourning in the Vatican and signals the start of a millennia-old process of picking a new pontiff.

It is a procedure steeped in tradition, but one which has been subtly updated for the modern world.

Cardinals — senior church officials appointed by the pope — from around the world must gather for the conclave in which Francis’ successor is selected. It typically takes between two and three weeks for a pope to be chosen, though it can stretch slightly beyond that if cardinals struggle to agree on a candidate.

The voting process is kept secret but will take place with the eyes of the world on the Vatican and amid intense scrutiny of the Catholic Church — an institution whose reputation has been stained by the scandal of child sex abuse within its ranks, overshadowing the legacies of successive popes.

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The period between pontiffs is known as the “papal interregnum,” when the church is without a spiritual leader. During this period of “sede vacante,” or “empty chair,” the church follows a clear set of rules as laid out in the apostolic constitution, Universi Dominici Gregis.