Semi-dome

A semi-dome, also called a "half-dome", is the term in architecture for half a dome ("cut" vertically), used to cover a semi-circular area. Similar structures occur in nature.

Semi-domes are a common feature of apses in Ancient Roman and traditional church architecture, and 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 (all shells were conches in Ancient Greek), 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 litugical east. The tetraconch, triconch and cross-in-square are other typically Eastern Christian church plans that produce several semi-domes.

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