An altarpiece is an artwork such as a painting, sculpture or relief representing a religious subject made for placing behind the altar of a Christian church.[1][2][3] 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 the Counter-Reformation

The usage and treatment of altarpieces were never formalised by the Catholic Church, and therefore their appearance can vary significantly.[5] Occasionally, the demarcation between what constitutes the altarpiece and what constitutes other forms of decoration can be unclear.[5] Altarpieces can still broadly be divided into two types, the reredos, which signifies a large and often complex wooden or stone altarpiece, and the retable, an altarpiece with panels either painted or with reliefs.[4] Retables are placed directly on the altar or on a surface behind it; a reredos typically rises from the floor.[4]

Retable-type altarpieces are often made up of two or more separate panels created using a technique known as panel painting. The panels can also display reliefs or sculpture in the round, either polychrome or un-painted. It is then called a diptych, triptych or polyptych for two, three, and multiple panels respectively. In the thirteenth century each panel was usually surmounted with a pinnacle, but during the Renaissance, single panel, or pala, altarpieces became the norm. In both cases the supporting plinth, or predella often featured supplementary and related paintings.

https://en.wikipedia.org/wiki/Altarpiece

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Note added at 28 mins (2017-04-01 19:09:15 GMT)
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W trakcie prac renowacyjnych okazało się, że ołtarze boczne mają podwójne obrazy. Jeden jest ruchomy i po zasunięciu przysłania obraz stały w ołtarzu.

http://www.mniszkow.pl/strona-333-parafia_w_blogiem_szlachec...