voussoir

thumb|William Henry Playfair, [[University of Edinburgh: bevelled edges of each stone block emphasise the voussoirs, which have a curved base and together form a semi-circle at the top of each arch.]] A voussoir (UK: ; US: ) is a wedge-shaped element, typically a stone, which is used in building an arch or vault. thumb|Joggled voussoir arch, located at the Mausoleum of Theodoric in [[Ravenna, Italy. (520 CE)]] Each voussoir is formed in a specific way to fit within the arch or vault created. Normally, different colored stones are shaped to fit together in a curved way that relies on the balance of each other to stay in place. The shaping of the individual wedge pieces varies depending on the type of arch being constructed—round, pointed, horseshoe, multifoil (which require voussoirs to be cut at different angles), and joggled. Joggled voussoirs were originally developed in Roman architecture to strengthen the structure, before being implemented in Islamic architecture. The wedged pieces have a curved shape, almost resembling puzzle pieces that fit together and lock into place. Joggled voussoirs were adopted by Islamic architects in order to strengthen arches and bridges by allowing them to sway with the movements of potential earthquakes and move back to their original position.

Although each unit in an arch or vault is a voussoir, there is distinct functional importance in two positions. Springers are 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. Springers often require precise shaping to control the direction of thrust and ensure successful transfer of weight to the supports below. The keystone is the center stone or masonry unit at the apex of an arch that locks the voussoirs in place. thumb|Keystone arch, Italian courtyard, 16th-century late Renaissance (Mannerism). 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). Taller keystones add a sense of stability, allowing the arches to be constructed more easily, with the keystone being dropped into the center of the arch. This central point on the arch balances the structure, but also starts an important identifying factor of Islam architecture and artistry. Many times throughout history, keystones were carved decoratively to highlight the central piece, usually with vegetal designs or inscriptions. In many architectural traditions, the keystone became a location for ornamental carving because of its visually prominent position. thumb|A single voussoir unit, made of marble, 6 5/8 x 14 1/2 in. (16.8 x 36.8 cm) 1130–40. 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 buttresses, piers or abutments. Voussoir arches distribute weight efficiently, and take maximum advantage of the compressive strength of stone, as in an arch bridge. The outer boundary of a voussoir forms an extrados, internal - an intrados (soffit).