A voussoir is a wedge-shaped element, typically a stone, which is used in building an arch or vault.

Although each unit in an arch or vault is a voussoir, two units are of distinct functional importance: the keystone and the springer. The keystone is the centre stone or masonry unit at the apex of an arch. The springer is the lowest voussoir on each side, located where the curve of the arch springs from the vertical support or abutment of the wall or pier.

The keystone is often decorated or enlarged. An enlarged and sometimes slightly dropped keystone is often found in Mannerist arches of the 16th century, beginning with the works of Giulio Romano, who also began the fashion for using voussoirs above rectangular openings, rather than a lintel (Palazzo Stati Maccarani, Rome, circa 1522).

The word is a mason’s term borrowed in Middle English from French verbs connoting a ‘turn’ (OED). Each wedge-shaped voussoir turns aside the thrust of the mass above, transferring it from stone to stone to the springer’s bottom face (‘impost’), which is horizontal and passes the thrust on to the supports. Voussoir arches distribute weight efficiently, and take maximum advantage of the compressive strength of stone, as in an arch bridge.

In Visigothic and Moorish architectural traditions, the voussoirs are often in alternating colours, usually red and white. This is sometimes found in Romanesque architecture also.

During the 18th and 19th centuries, British bricklayers became aware that, by thickening the vertical mortar joint between regularly shaped bricks from bottom to top, they could construct an elliptical arch of useful strength over either a standard ‘former’, or over specially constructed timber falsework (temporary structure to be removed once the construction is complete). The bricks used in such an arch are often referred to as ‘voussoirs’.

Today, this term is used as synonym of claveau (synonym already used in the 13th century by Villard de Honnecourt, in his Notebook, but the following centuries have privileged the distinction), whereas formerly, the claveau was only part of of the arch or the flower bed, and the voussoir of the vault, as their etymology confirms.

Characteristics
We distinguish:

to go extradossed: voussoir whose head is level and which forms the extrados of the vault;
to go to the branch: to see who has two branches in fork to make connection with the pendant of a vault of ridge;
crossette: voussoir whose upper part is at an angle to connect with a level seat.
More contemporary, the voussoirs are made of concrete. Prefabricated then assembled on the work of art, they form the vaults of tunnels or arcades of bridges and viaducts.

Elements of the arc from the culex
The lowest located wedge in the arch or vault is called the base, while the wedge is the top wedge, the middle is a keystone. The most powerful wedge is usually placed in the base (where the greatest force works) and at the top of the arch (it takes up strength from above and can break or fall out). These elements are almost always made of stone. The remaining blades are subjected to mutual compression. Intermediate stubs can be made of bricks.

peak gown – keystone
intermediate wedge
ridge
socket wrench
Under the arch
height (arrow) of the arch
range
armpit – filling

Source From Wikipedia