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One of the wedge-shaped stones forming the curved parts of an arch or vaulted ceiling.
(Architecture) a wedge-shaped stone or brick that is used with others to construct an arch or vault
[C18: from French, from Vulgar Latin volsōrium (unattested), ultimately from Latin volvere to turn, roll]
any of the pieces in the shape of a truncated wedge that form an arch or vault.
[1325–75; < French; replacing Middle English vousor(i)e < Anglo-French; Old French volsoir < Vulgar Latin *volsōrium=*volt(us) (for Latin volūtus), past participle of volvere to turn + -tōrium -tory2]