Winged altarpiece
A winged altarpiece (also folding altar) or winged retable is a special form of altarpiece (reredos, occasionally retable), common in Northern and Central Europe, in which the central image, either a painting or relief sculpture (or some combination of the two) can be hidden by hinged wings. It is called a triptych if there are two wings, a pentaptych (but this is rarely used in English) if there are four, or a polyptych if there are four or more.