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 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 arches of the 16th century, such as the portal of the "church house" at Colditz Castle. The springer is the lowermost voussoir, located where the curve of the arch springs from the vertical support or abutment of the wall or pier.
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 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 falsework (temporary structure to be removed once the construction is complete). The bricks used in such an arch are often referred to as 'voussoirs'.
- "Glossary of Medieval Art and Architecture - Voussoir". University of Pittsburgh. Retrieved 2007-06-24.
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