Arch

An arch is a curved vertical structure that spans an open space, made of wedge shaped blocks called voussoirs held together in compression, with the topmost voussoir forming the keystone. As a decorative element the arch dates back to the fourth millennium BC, but structural load bearing arches became widespread only after the Ancient Romans adopted them from the fourth century BC onward. An arch has a real advantage over a simple lintel: for the same amount of material an arch can span a larger opening, carry more weight, and be built from smaller, more manageable pieces, because the load travels in compression around the curve rather than in bending across one beam. Extensive use of arches and vaults is called arcuated construction, contrasted with the trabeated, post and beam system used in ancient Greece, China and Japan. The arch's dominance in load bearing masonry construction declined in the mid nineteenth century once wrought iron, and later steel, made long, high tensile lintels possible.