Ecoinçon (spandrel)

An écoinçon is the space between two arcs (arches) or between an arch and a rectangular envelope.
The top picture shows clearly what a spandrel is; it makes it easier to spot the spandrels in the lower picture.
The photograph shows a detail of one of the clochers (bell towers) of the Saint-Philibert in Tournus.

Ëcoinçon - Wikipedia

Saint-Philibert - Tournus