Claustra (claustra ©)

The translation in the title is followed by this sign ©; this means that it has been "invented" by me.

A number of ajouré (ajoure) tiles closing off a baie (opening).
In Rome I stumbled upon an impressive collection of claustras in the Basilica di Santa Sabina.
In my own collection I also found some of these tiles which have been used as decoration in the walls of one of the chapelles (chapels) of the Saint-Philibert in Tournus.

Basilica di Santa Sabina - Rome

Saint-Philibert in Tournus

Claveau (voussoir)

Wedge shaped stone used to build an arc (arch) or voûte (vault).
The picture comes from Wikipedia.
See also voussoir (synonym).

Claveau - Wikipedia

Claveau droit (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.
See also clef de voûte (synonym).

Claveau droit - Wikipedia

Lauze (flagstone)

Laves, as they are called in Burgundy, or lauzes, as they are called elsewhere, are flat stones of shale or limestone, used as floor or roofing tiles.
The roofing of the chevet of the Notre-Dame in Chazelle (Cormatin) consists of lauzes.
See also lave (synonym).

Notre-Dame - Chazelle (Cormatin)

Lave (flagstone)

Laves, as they are called in Burgundy, or lauzes, as they are called elsewhere, are flat stones of shale or limestone, used as floor or roofing tiles.
The roofing of the chevet of the Notre-Dame in Chazelle (Cormatin) consists of laves.
See also lauze (synonym).

Notre-Dame - Chazelle (Cormatin)

Moellon (building block)

A crudely worked piece of stone, often square.
Since the words Romanesque and Roman are related, this photo showing the moellons in the wall of the théâtre antique of Orange (84) around the sculpture of Augustus is not completely out of place.

Théâtre antique - Orange

Monolithe (monolithic)

A sculpture made from one piece of stone.
A colonne (column) can also be hewn from one piece of stone; in that case it is called a monolithic column.
The word monolithe (monolith) can also be just a solid piece of stone, like those lining Stone Henge. This monolithe can be found in a field near La Chapelle-sous-Brancion.

Menhir - La Chapelle-sous-Brancion

Sommier (springer)

The first stone of an arc (arch), resting on the top of a pilier (pillar).
The sketch (Wikipedia) shows clearly which stones are the sommiers.

Sommier - Wikipedia