ARCH, any combination of blocks of building material, gen erally wedge-shaped and with radial joints, employed to cap an opening wider than any of the blocks themselves capping it. In form, arches are usually, though not always, built with the soffit (q.v.) following a curved line. By extension, the word arch is used for any curved head of an opening or recess, even when the material is homogeneous, as in a concrete arch. From the use of arch forms, to bridge the spaces between the beams in early fireproof construction, the word arch is employed technically for any structure between steel beams, even when the structure may be of reinforced concrete, and, therefore, theoretically a beam, and not an arch at all.
In the normal arch, the inside face or soffit is known as the intrados, the outside face as the extrados, the wedge-shaped blocks as voussoirs, the centre voussoir as the keystone and the two end voussoirs as the springers. The spring of the arch is the level of the bottom of the springers, which usually coincides with the beginning of the curvature, but a stilted arch is one in which the apparent spring is well below this beginning. The haunches of an arch are the parts between keystone and springer. A con tinuous arch, such as a tunnel, is known as a vault.
Due to the nature of its construction, with wedge-shaped blocks, any arch exerts at its spring, not only a downward weight, but a tendency to spread, which is known as thrust, and for the arch to remain stable it is necessary for this thrust to be resisted ade quately by abutments, buttresses or the strength of the wall itself in which the arch is placed. This quality of exerting thrust has profoundly affected architecture (see BUTTRESS, FLYING BUT