A voussoir is a wedge-shaped element, typically a stone, used in building an arch.
Although each unit of stone in an arch or vault is known as a voussoir, there are two specified voussoir components of an arch: the keystone and the springer. The keystone is the center stone or masonry unit at the apex of an arch, often decorated, embellished or exaggerated in size. No true arching action occurs until this unit is in place. The springer is the lowermost voussoir, located where the curve of the arch springs from the vertical support or abuttment 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 final edge, which is horizontal and passes the thrust to the supports. Voussoir arches distribute weight efficiently and take maximum advantage of the compressive strength of stone, as in an arch bridge. The outer boundaries of a voussoir are the extrados.