In 1621 Guercino’s Bolognese patron Alessandro Ludovisi was elected Pope Gregory XV, and Guercino followed him to Rome. Gregory commissioned from Guercino a huge altarpiece, to be placed over the remains of St Petronilla in St Peter’s. The young artist must have prepared this model drawing for the approval of his patron; approval was evidently not forthcoming, for the altarpiece is very different in its composition.