An altarpiece is a painting or sculpture, including relief, of religious subject matter made for placing at the back of or behind the altar of a Christian church. Though most commonly used for a single work of art such as a painting or sculpture, or a set of them, the word can also be used of the whole ensemble behind an altar, otherwise known as a reredos, including what is often an elaborate frame for the central image or images. Altarpieces were one of the most important products of Christian art especially from the late Middle Ages to the era of Baroque painting.
The word altarpiece, used for paintings, usually means a framed work of panel painting on wood, or later on canvas. In the Middle Ages they were generally the largest genre for these formats. Murals in fresco tend to cover larger surfaces. The largest painted altarpieces developed complicated structures, especially winged altarpieces with hinged side wings that folded in to cover the main image, and were painted on the reverse with different simpler images. Often this was the normal view shown in the church, except for Sundays and feast days, when the wings were opened to display the main image. At other times visitors could usually see this by paying the sacristan.
