Krakow High Altar in the context of Altar (Catholicism)

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. The technical terms are derived from Ancient Greek: τρίς: trís or "triple"; πέντε: pénte or "five"; πολύς: polýs or "many"; and πτυχή: ptychē or "fold, layer".

There are often images on both the insides and outsides of the wings, enabling the altarpiece to display completely different views when open and closed. It was usually the custom to keep the wings closed except on Sundays or feast days, although very often the sacristan would open them for tourists at any time for a modest tip. Small winged paintings, usually triptychs, were also owned by the wealthy for private devotions, and services in the house; they had the advantage that the open view was fairly well protected when covered up during travel.