A study for the heads of the Saint William, the Bishop and two acolytes for the altarpiece painted in 1620 for San Gregorio, Bologna and now in the Pinacoteca there, L. Salerno, I Dipinti del Guercino, Rome, 1988, no. 69. The commission was the most important Guercino received before his departure to Rome in 1621. Guercino's careful preparation for the painting is clear from the large number of related drawings, see D. Mahon, Il Guercino: Catalogo critico dei disegni, Bologna, 1969, nos. 57-76 and N. Turner and C. Plazzotta, op. cit., nos. 26-8