In architecture, a structure built of separate and rigid blocks shaped like wedges and put together on a curved line so as to keep their position by mutual pressure when the arch is supported only at its two ends. The separate blocks are called voussoirs or arch-stones, the lowest members of which are termed springers, and the uppermost or central one, when a single stone is thus used, is called the keystone. The under or concave side of the voussoir is called the intrados or soffit; the upper or convex, the extrados. The supports of the arch are called piers, pillars, and abutments, the two former receiving the vertical pressure and the latter resisting the lateral thrust. The upper part of the pier is the impost, the span being the distance between the opposite imposts, above the line of which the highest point of the intrados is called the rise. The thrust is the pressure exerted outward, and is counteracted by the abutments or buttresses. Of the various types exemplified in the illustration the pointed arch is stronger than any other kind.

Bibliography Information
Entry for 'Arch'. 1910 New Catholic Dictionary. https://www.studylight.org/dictionaries/eng/ncd/a/arch.html. 1910.

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