Pendentif (pendentive)

Elements of a sphere, enabling the transformation from a square base to a circular coupole (dome).
This is a clear case where one picture (Wikipedia) says more than a thousand words!
The pendentives are marked with the letter "a".
The picture shows the pendentives with trompes (squinches) in the Notre-Dame-de-l’Assomption in Ciel.

Pendentif - Wikipedia

Notre-Dame-de-l’Assomption - Ciel