A voussoir (pronounced /vuˈswɑr/) is a wedge-shaped element,
typically a stone, 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 center stone or masonry unit
at the apex of an arch.
- 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 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'.
_-_Dettaglio_-_Foto_Giovanni_Dall%27Orto,_31-Aug-2007.jpg "Decorative Lion Head Keystone-image courtesy Wikipedia")
