Because most triptych altarpieces are usually kept unopened, the Portinari Altarpiece would have been closed except for special occasions such as holidays and feast days when it would have been opened. The exterior of Hugo van der Goes's triptych has decorative depictions of the Annunciation scene. The artist Hugo van der Goes painted the figures of the Virgin Mary and of the Angel Gabriel in grisaille, meaning to paint it in a style that imitates sculpture. The two figures are placed in shallow niches. During the completion of this work in the Netherlands, to depict grisaille on the exterior of altarpieces was a tradition for artists. This tradition came from the Middle Ages, when sculpture was considered the chief medium for devotional art. In the fifteenth and sixteenth centuries, some places in Germany continued to see sculpture as superior and that it belonged in the middle of a folding altarpiece like the one by Van der Goes.