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

Arch

An arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...

 or vault.

Although each unit in an arch or vault is a voussoir, two units are of distinct functional importance: the keystone

Keystone (architecture)

A keystone is the wedge-shaped stone piece at the apex of a masonry vault or arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight. This makes a keystone very important structurally...

 and the springer

Springer (architecture)

A springer is an architectural term for the lowest voussoir on each side of an arch. Since it is the bottom-most element of the arch, it is where the arch support terminates at the responds. It rests on the impost or pier of the arch, that is, the topmost part of the abutment, from which the arch...

. The keystone is the center stone or masonry unit at the apex of an arch. (It is often decorated or enlarged. An enlarged and slightly dropped keystone is often found in Mannerist

Mannerism

Mannerism is a period of European art that emerged from the later years of the Italian High Renaissance around 1520. It lasted until about 1580 in Italy, when a more Baroque style began to replace it, but Northern Mannerism continued into the early 17th century throughout much of Europe...

 arches of the 16th century, such as the portal of the "church house" at Colditz Castle

Colditz Castle

Colditz Castle is a Renaissance castle in the town of Colditz near Leipzig, Dresden, and Chemnitz in the state of Saxony in Germany. Used as a workhouse for the indigent and a mental institution for over 100 years, it gained international fame as a prisoner-of-war camp during World War II for...

.) The springer is the lowermost voussoir, located where the curve of the arch springs from the vertical support or abutment

Abutment

An abutment is, generally, the point where two structures or objects meet. This word comes from the verb abut, which means adjoin or having common boundary. An abutment is an engineering term that describes a structure located at the ends of a bridge, where the bridge slab adjoins the approaching...

 of the wall or pier

Pier (architecture)

In architecture, a pier is an upright support for a superstructure, such as an arch or bridge. Sections of wall between openings function as piers. The simplest cross section of the pier is square, or rectangular, although other shapes are also common, such as the richly articulated piers of Donato...

.

The word is a mason's term borrowed in Middle English

Middle English

Middle English is the stage in the history of the English language during the High and Late Middle Ages, or roughly during the four centuries between the late 11th and the late 15th century....

 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

Compressive strength

Compressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g...

 of stone, as in an arch bridge

Arch bridge

An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side...

.

In Eastern Romanesque and Arab architecture the voussoirs are often in alternating colors, usually red and white.

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 false work, (work to be removed following the construction of the prime). The bricks used in such an arch are often referred to as 'voussoirs'.