One hundred and thirty-three cardinals are sequestering themselves behind the Vatican’s medieval walls for the start of a conclave to elect a successor to Pope Francis.

Francis’ successor will be selected by cardinals at the conclave that begins on May 7. Cardinals, known as the “princes of the church,” rank just below the pope in the Roman Catholic Church; together, they are known as the College of Cardinals. When a pope dies, the college chooses a successor.