Clef de voûte (keystone)

The highest stone of an arc (arch) or voûte (vault).
In a vault the keystone is normally located at the crossing of the ogives (ribs) of the vault, and often beautifully decorated.
A well known keystone is one that was rescued from Cluny III (see picture).
N.B. : The French are using two different spellings for the word key : clef and clé.
Wikipedia uses both spellings at random.
See also claveau and claveau droit (synonym).

Musée Ochier - Cluny