The Crucifixion Altarpiece is a triptych formed of three painted panels, the two side wings belonging to the Walker Art Gallery and the central painting to the National Gallery, London. The entire triptych is presently on display in the Walker. Both wings have painted reverses which would be folded closed in front of the central panel to show a family witnessing the 'Mass of St Gregory'. The wings were separated from the centre panel in the early 19th-century before or during their export to Britain. Until 1963 the paintings on the backs of the wings were hidden under black paint and only rediscovered after cleaning and conservation.