Trompe (squinch)

A trompe is a construction element, build out from the corner of a square space to facilitate the transition from a square to a circular or octagonal area, which in its turn is closed off with a coupole (cupola).
This rather cryptic description is hopefully clarified by the picture.
In the picture the four trompes having the shape of a quarter-circle, supporting the coupole under the clocher (bell tower) of the Saint-Pierre-aux-Liens in Curbigny are clearly visible.

Saint-Pierre-aux-Liens - Curbigny