Semi-domes are a common feature of apses in Ancient Roman and traditional church architecture, and in mosques and iwans in Islamic architecture. A semi-dome, or the whole apse, may also be called a conch after the scallop shell often carved as decoration of the semi-dome, though this is usually used for subsidiary semi-domes, rather than the one over the main apse. Small semi-domes have been often decorated in a shell shape from ancient times, as in Piero della Francesca's Throned Madonna with saints and Federigo da Montefeltro, and the example in the gallery below. Islamic examples may use muqarnas decorative corbelling, while in Late Antique, Byzantine and medieval church architecture the semi-dome is the classic location for a focal mosaic, or later fresco. Found in many Ancient Greek exedras, the semi-dome became a common feature of the apse at the end of Ancient Roman secular basilicas, which was adopted in Early Christian architecture as the commonest shape for churches, becoming the focal point for decoration. In buildings like Hagia Sophia in Byzantine architecture, apsidal openings or exhedras from the central nave appear in several directions, not just to the liturgical east. The tetraconch, triconch and cross-in-square are other typically Eastern Christian churchplans that produce several semi-domes. When the Byzantine styles were adapted in Ottoman architecture, which was even less concerned with maintaining a central axis, a multiplicity of domes and semi-domes becomes the dominating feature of both the internal space and the external appearance of the building. The buildings of Mimar Sinan and his pupil Sedefkar Mehmed Agha are the masterpieces of this style. Mihrabs are another common location for semi-domes. In Western Europe the external appearance of a semi-dome is less often exploited than in Byzantine and Ottoman architecture, and is often disguised as a sloping rather than curved semi-circular roof. Cincinnati Union Terminal in Cincinnati, Ohio features the largest semi-dome in the Western hemisphere, measuring 180 feet wide and 106 feet high.
