Arc diaphragme (diaphragm arch)

This is a transverse wall-bearing arc (arch) forming a partial wall, dividing a voûte (vault) or a plafond (ceiling) into compartments.
The picture (from internet) shows an arc diaphragme in an oratoir carolingien in Germigny-des-Prés (Loiret 45).
The picture underneath shows the arcs diaphragmes (from left to right) in the Saint-Philibert in Tournus.

Oratoire Carolingien - Germigny-des-Prés

Saint-Philibert - Tournus