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.