Arch, a self-supporting span, usually curved, across an opening. The arch is used in architecture and in bridge building.

The wedge-shaped pieces of a stone or brick arch are called voussoirs . The central voussoir is called the keystone. It is usually heavier than the other voussoirs and is often decorated with carving. The voussoirs are so arranged that they support each other and, together, the weight of the structure above them. This weight presses against the side supports (abutments or piers) of an arch, creating an outward force called thrust. The form of the arch is usually based on the circle, but many different styles exist.

The principle of the arch is used in the vault, an arched stone or brick ceiling. The barrel, or tunnel, vault resembles a continuous round arch. When two barel vaults interset at right angles a groin or cross, vault is formed. In a ribbed vault, a framework of ribs, or arches, provides additional support. Arcades and domes are also based on the arch.

Although known to the ancient Egyptians and Greeks, the arch was rarely used before the time of the Roman Empire. Roman architects used the round arch in aqueducts, public baths, amphitheaters, and other structures. During the Middle Ages the round arch was characteristic of Romanesque cathedrals and the pointed arch of Gothic cathedrals. Islamic architects favored the horseshoe, ogee, and multifoil arches.

Commemorative Arches

Commemorative arches are huge monuments built to honor important men or events. The Romans erected commemorative arches, called triumphal arches, to emperors and generals, celebrating military victories. The Arch of Titus, Trajan's Arch, and the Arch of Constantine are among the best-preserved Roman triumphal arches.

Although commemorative arches were built also during the Renaissance, the most famous arches since Roman times have been built during the last 200 years. Among them are the Arc de Triomphe in Paris, the Marble Arch in London, and the Washington Arch in New York City. Gateway Arch, a 630-foot (192-m) stainless-steel arch in St. Louis, Missouri, commemorates the Louisiana Purchase and the westward expansion of the United States.