Pendentive

In architecture, a pendentive is a constructional device permitting the placing of a circular dome over a square room or of an elliptical dome over a rectangular room.

[1] The pendentives, which are triangular segments of a sphere, taper to points at the bottom and spread at the top to establish the continuous circular or elliptical base needed for a dome.

Prior to the pendentive's development, builders used the device of corbelling or squinches in the corners of a room.

Pendentives commonly occurred in Orthodox, Renaissance, and Baroque churches, with a drum with windows often inserted between the pendentives and the dome.

The first experimentation with pendentives began with Roman dome construction in the 2nd–3rd century AD,[3] while full development of the form came in the 6th-century Eastern Roman cathedral, Hagia Sophia, in Constantinople (Istanbul).