Apse

An apse is a semicircular or polygonal recess, typically capped with a half dome or a half vault, projecting from the east end of a church beyond the line of the nave and transept, and it houses the altar and, in many traditions, the seat historically reserved for the presiding bishop or priest. The half-dome ceiling of the apse is frequently one of the most heavily decorated surfaces in the entire church, carrying mosaic or fresco imagery of Christ, the Virgin, or the patron saint on a scale and prominence matched by few other single surfaces in the building, since it sits directly above the altar and is the visual focus toward which the whole nave is oriented.

Because the apse projects outward beyond the main rectangular volume of the building, it is also often clearly visible from the church's exterior as a rounded or polygonal projection at the east end, giving many church footprints their easily recognizable combination of a long rectangular nave terminating in a curved apse. In churches with an ambulatory, the apse is typically encircled by this curved walkway, letting pilgrims pass behind and around the sanctuary, often past a ring of smaller radiating chapels, without crossing the sanctuary space itself.

Like the nave it terminates, the apse derives from the Roman basilica, where in its secular civic use the semicircular recess at one end of the hall held the magistrate's raised tribunal seat, the point from which legal judgments were formally delivered to those assembled in the hall before it. Christian builders repurposed this same architectural gesture for a very different kind of authority, placing the altar and the bishop's seat, the cathedra, in the position the magistrate had occupied, so the apse carried directly across from Roman civic architecture into Christian church design a spatial statement about where authoritative pronouncement happens within the building.

Over the following centuries the apse became the standard focus for a church's most ambitious decorative programs, and regional traditions developed distinctive treatments of the form: Byzantine and Eastern churches typically favor a single dominant apse mosaic of Christ Pantocrator or the Theotokos, Romanesque churches in the West often combine a rounded apse with a full ambulatory and radiating chapels for pilgrimage traffic, and later Gothic cathedrals sometimes replace the simple rounded apse with a polygonal chevet, a more complex east end integrating the apse, ambulatory and chapels into a single unified, heavily glazed architectural composition.