Sacristan in the context of "Ostiarius"

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.

A polyptych (/ˈpɒlɪptɪk/ POL-ip-tik; Greek: poly- "many" and ptychē "fold") is a work of art (usually a panel painting) which is divided into sections, or panels. Some definitions restrict "polyptych" to works with more than three sections: a diptych is a two-part work of art; a triptych is a three-part work; a tetraptych or quadriptych has four parts; a pentaptych has five parts. The great majority of historical examples are paintings with religious subjects, but in the 20th century the format became popular again for portraits and other subjects, in painting, photography, and other media.

Historically, polyptychs were panel paintings that typically displayed one "central" or "main" panel that was usually the largest; the other panels are called "side" panels, or if hinged, "wings". Folding forms were much more common north of the Alps. Sometimes, as evident in the Ghent Altarpiece and Isenheim Altarpiece, the hinged panels can be varied in arrangement to show different "views" or "openings" in the piece, because the wing panels are painted with images on both sides. The wings were usually kept folded shut, showing the "closed" view, except on Sundays or feast-days, or if visitors paid the sacristan for a sight of the "open" view. The upper panels often depict static scenes, while the lower register, the predella, often depict small narrative scenes.

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.