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thumb|Typical early Christian Byzantine apse with a hemispherical semi-dome in the [[Basilica of Sant'Apollinare in Classe]] thumb|Typical floor plan of a cathedral, with the apse shaded In architecture, an apse (: apses; from Latin , 'arch, vault'; from Ancient Greek , , 'arch'; sometimes written apsis; : apsides) is a semicircular recess covered with a hemispherical vault or semi-dome, also known as an exedra. In Byzantine, Romanesque, and Gothic Christian church (including cathedral and abbey) architecture, the term is applied to a semi-circular or polygonal termination of the main building at the liturgical east end (where the altar is), regardless of the shape of the roof, which may be flat, sloping, domed, or hemispherical. Smaller apses are found elsewhere, especially in shrines.

==Definition== An apse is a semicircular recess, often covered with a hemispherical vault. Commonly, the apse of a church, cathedral or basilica is the semicircular or polygonal termination to the choir or sanctuary, or sometimes at the end of an aisle.