arch

In masonry, a curved structure that supports the weight of material over an open space, as in a bridge or doorway. The first arches consisted of several wedge‐shaped stones supported by their mutual pressure. The term is also applied to any curved structure that is an arch in form only, such as the Arc de Triomphe, Paris, 1806–36.

The Romans are credited with engineering the earliest round keystone arches, used for aqueducts. Other forms of arch include the pointed arch, the corbelled arch of the Maya Indians, the medieval lancet and ogee arches, and the Islamic horseshoe arch.

History
The principle of the arch was known to the ancient Egyptians, Mesopotamians, and Greeks but arches were seldom used until Roman times, when the semicircular arch was universally adopted. This form continued in fashion during the phases of architecture known as Romanesque in Western Europe, and Byzantine in Eastern Europe. Pointed arches, first used in India about the 6th century BC, were employed in Mesopotamia and were adopted by early Muslim architects; they were introduced into England and France, probably by returning crusaders, at the end of the 11th century. In Muslim countries, mainly south of the Mediterranean, the horseshoe arch, sometimes round and sometimes pointed, was popular. In Renaissance architecture, throughout Western Europe, round arches replaced the pointed type.

Terms and parts
The principal terms connected with any form of arch are the span, rise, and springing line; and the various parts: abutment, archivolt (moulding around the face or underside of an arch), crown, extrados, impost, intrados, keystone, spandrel, springer, and voussoir.

Types
The principal types of arch, according to their shape, are semicircular or ‘round’, segmental, stilted, round horseshoe, equilateral (pointed, with the radii equal to the span), lancet (pointed, with radii larger than the span), four‐centred (pointed, with four arcs), pointed horseshoe, three‐centred, cusped, and elliptical.