Déambulatoire (ambulatory)

A corridor running along the chœur (choir) which starts and ends at the bas-côtés (aisles).
The plan shows the ambulatory in grey.
The purpose of this corridor was to enable a procession to walk past the reliques (relics).
In the picture the ambulatory in the Notre-Dame-de-la-Nativité in Bois-Sainte-Marie, as seen from the choir.

Déambulatoire - Wikipedia

Notre-Dame-de-la-Nativité - Bois-Sainte-Marie