This article is about an architectural feature; for the astronomical term see apsis.
In Romanesque, Byzantine and Gothic Christian abbey, cathedral and church architecture, the apse (Latin absis "arch, vault"; sometimes written apsis; plural apses) is the semi-circular or polygonal section of the sanctuary at the liturgical east end beyond the altar (plan, right). The semicircular projection (which may be polygonal on the exterior, or reveal the radiating projections of chapels) may be roofed with a half-dome or with radiating vaulting. A simple apse may be merely embedded within the wall of the east end. Eastern orthodox churches may have a triple apse, which is usually a mark of Byzantine influence when it is seen in Western churches. Smaller subsidiary apses may be found around the choir or even at the ends of transepts.
Compare the exedra of Classical architecture, which may be considered apsidal, a feature of the secular Roman basilica, which provided some prototypes for Early Christian churches. The apse in the Roman basilica was often raised (as the sanctuary generally still is) as a hieratic feature that set apart the magistrates who deliberated within it.
A simple apse set into the east end of an English parish church, at Poulton_le_Fylde, Lancashire
An exedra or apse may be reduced in scale to form a niche within the thickness of walling; a niche does not reveal its presence by projecting on the exterior.
The interior of the apse is traditionally a focus of iconography, bearing the richest concentration of mosaics, or painting and sculpture, towards which all other decoration may tend.
Various architectural features that may form part of the apse are drawn together here:
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In the presbytery or sanctuary directly to the east beyond the choir is the High Altar, where there is one (Compare communion table. This area is reserved for the clergy.
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In the beginning of the 13th century in France, the apses were built as radiating chapels outside the choir aisle, henceforth known as the chevet (French, "headpiece"), when the resulting structure was too complicated to be merely an "apse." Famous northern French examples of chevets are in the Gothic cathedrals of Amiens, Beauvais and Reims. Such radiating chapels are found in England in Norwich and Canterbury cathedrals, but the fully_developed feature is essentially French, though the Francophil connoisseur Henry III introduced it into Westminster Abbey.
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