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An arch is a vertical curved structure that spans an elevated space and may or may not support the weight above it, or in case of a horizontal arch like an arch dam , the hydrostatic pressure against it. Arches may be synonymous with vaults, but a vault may be distinguished as a continuous arch forming a roof . Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture , and their systematic use started with the ancient Romans, who were the first to apply the technique to a wide range of structures.