Pendentive

A pendentive is a triangular curved masonry element that makes it possible to set a circular dome over a square room, or an elliptical dome over a rectangular one. Each pendentive is a segment of a sphere that comes to a point at the bottom, where it lands on one of the four corner piers, and spreads outward at the top until the four pendentives together form the continuous circular or elliptical base a dome needs to rest on; in this way the pendentives gather the dome's weight and carry it down to the piers beneath. Before builders adopted the pendentive, the corners of a square room were bridged instead with corbelling or with squinches, simpler diagonal elements that make the same transition less smoothly. Roman builders began experimenting with pendentive construction in domes of the second and third centuries AD, but the form reached its fullest expression in the sixth century Eastern Roman cathedral of Hagia Sophia in Constantinople, and pendentives went on to recur constantly in Orthodox, Renaissance and Baroque church design, often with a windowed drum inserted between the pendentives and the dome above them.