Pendentives support Hagia Sophia’s circular dome over a square central space.
A pendentive is a curved triangular section of masonry that transitions from the corners of a square bay to the circular base of a dome. Four pendentives together create a continuous ring on which the dome can rest. This allowed Byzantine builders to place a large dome above a centralized or square-plan space without using a circular drum as the primary transition.
Hagia Sophia’s design made especially influential use of this technique. The dome rises above four massive piers, while the pendentives visually dissolve the corners and help create the famous impression of a floating canopy.
Pendentives are not the same as squinches. A squinch uses an arch, corbel, or small vault across a corner, while a pendentive is a continuous curved surface. The distinction matters because both devices solve the same basic geometric problem in different ways.