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<p>An arch is a curved vertical <a href="/facts/Structure/xQDPtqDQ">structure</a> <a href="/facts/Span_(engineering)/NqBN05al">spanning</a> an open space underneath it. Arches may support the load above them, or they may perform a purely decorative role. As a decorative element, the arch dates back to the <a href="/facts/4th_millennium_BC/IRr7dCr1">4th millennium BC</a>, but structural load-bearing arches became popular only after their adoption by the <a href="/facts/Ancient_Romans/cGqIuxPd">Ancient Romans</a> in the 4th century <a href="/facts/Anno_Domini/NmQwnQNj">BC</a>. </p><p>Arch-like structures can be horizontal, like an <a href="/facts/Arch_dam/A42zfPY9">arch dam</a> that withstands the horizontal <a href="/facts/Hydrostatic_pressure/GYu6ljQ0">hydrostatic pressure</a> load. Arches are usually used as supports for many types of <a href="/facts/Vault_(architecture)/HjLDot2V">vaults</a>, with the <a href="/facts/Barrel_vault/owxuMAwF">barrel vault</a> in particular being a continuous arch. Extensive use of arches and vaults characterizes an arcuated construction, as opposed to the <a href="/facts/Trabeated/qkFbJoa6">trabeated</a> system, where, like in the architectures of ancient Greece, China, and Japan (as well as the modern <a href="/facts/Steel-framed/jNHlGJtx">steel-framed</a> technique), <a href="/facts/Post_and_lintel/qkFbJoa6">posts and beams</a> dominate. </p><p>Arches had several advantages over the <a href="/facts/Lintel/JtoKkefa">lintel</a>, especially in the <a href="/facts/Masonry/TlaDzpUp">masonry</a> construction: with the same amount of material it can have larger span, carry more weight, and can be made from smaller and thus more manageable pieces. Their role in construction was diminished in the middle of the 19th century with introduction of the <a href="/facts/Wrought_iron/zynpjrIv">wrought iron</a> (and later <a href="/facts/Steel/aN6qWo4H">steel</a>): the high <a href="/facts/Tensile_strength/Cv2jPplR">tensile strength</a> of these new materials made long lintels possible. </p>