In architecture, an apse is a semicircular recess covered with a hemispherical vault or semi-dome, also known as an exedra. In Byzantine, Romanesque, and Gothic Christian church architecture, the term is applied to a semi-circular or polygonal termination of the main building at the liturgical east end, regardless of the shape of the roof, which may be flat, sloping, domed, or hemispherical. Smaller apses are found elsewhere, especially in shrines.

In architecture, a vault is a self-supporting arched form, usually of stone or brick, serving to cover a space with a ceiling or roof. As in building an arch, a temporary support is needed while rings of voussoirs are constructed and the rings placed in position. Until the topmost voussoir, the keystone, is positioned, the vault is not self-supporting. Where timber is easily obtained, this temporary support is provided by centering consisting of a framed truss with a semicircular or segmental head, which supports the voussoirs until the ring of the whole arch is completed.