Pilastre (pilaster)

A pilaster is a rectangular wall strengthening, protruding less than its width from the wall.
The tall strengthenings next to the louvre-holes are pilasters, as are the short ones underneath the louvre-holes.
The picture shows a pilastre in the abside (apse) of the Saint-Pierre-aux-Liens in Curbigny.

Saint-Pierre-aux-Liens - Curbigny