Pendentive

A pendentive is a curved triangular section of masonry that provides the structural transition from a square or polygonal room below to a circular dome above. Each pendentive is a segment of a larger sphere, positioned so that its four points meet the corners of the supporting walls or piers, its curved sides rise to meet the neighboring pendentives, and its top forms a level circle that supports the base of the dome, letting a round dome sit over a room without needing an equally round base beneath it. The technique, realized on a monumental scale in the sixth-century dome of the Hagia Sophia, became a standard method for placing a dome over a square plan in Byzantine, Islamic and later Western architecture.