Petronilla, whose name means "little rock", is popularly believed to have been the daughter of Saint Peter, whose Greek name, "Petros", means "rock". Her relics had rested in the catacombs of Domitilla in Rome until 757 when Pope Paul I translated her body to St Peter’s in the circular mausoleum called thereafter of St Petronilla. The saint was named by the same Pope protector and patron of the French Kings to reward Pepin the Short of his service to the Papacy in front of the Lombard invasion. When the Rotunda of St Petronilla was demolished during the construction of the new St Peter, her body was translated to a new altar within the new church (1606). Guercino was instructed then to paint an altarpiece for the altar above her tomb.