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.
The form was especially popular in the later Middle Ages, and during the Northern Renaissance. In the 17th century, Rubens was one of the last major painters to use it. It was never as popular in Italy, where there were many polyptychs, but usually built without hinges, so always "open", even if there were also images on the back, as in the Maestà by Duccio for Siena Cathedral.
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