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Arch, structural device to span openings and support loads. In architecture the simplest form of the arch is the round (semicircular), in which wedge-shaped stones, or voussoirs, are fitted together, receiving the stresses in the arch exerted outward onto them. Downward forces from the load combine with these to produce a diagonal force, or thrust. The voussoirs at each end of the arch are called springers; the one in the center, usually the last to be placed, is the keystone. Although the arch was known in ancient Egypt and Greece, it was not until Roman times that its use became popular.

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