A vertical reinforcement in stone of a wall, in the shape of a pilastre (pilaster) on one side of the wall, level with the wall on the other side.
On the level side of the wall the strengthening looks like a chain, hence the name.
The sketch came from Wikipedia.
A chaîne is often used on corners of buildings.
In the picture a chain pattern on the corner of the Notre-Dame in Clessé.
|Chaîne - Wikipedia|
|Notre-Dame - Clessé|