Painting, relief, sculpture, screen, or decorated wall standing on or behind an altar in a Christian church. The images depict holy personages, saints, and biblical subjects. There are two types of altarpieces: the reredos, which rises from the floor behind the altar, and the retable, which stands on the altar itself or on a pedestal behind it. The diptych is an altarpiece consisting of two panels; a triptych, three panels; and a polyptych, four or more panels. Altarpieces vary in size; some are small and portable, some are huge and stationary, and some have movable wings that can be opened and closed. The practice of erecting sculptural altarpieces dates from the 11th century; altar paintings became common in the 14th century.