A flying buttress; an external curved stone support structure that extends from the wall of a building to help support the roof or vault from outside.
From French 'arc boutant', where 'arc' means 'arch' and 'boutant' means 'thrusting' or 'pushing'. The term literally describes what the structure does—it thrusts support outward to catch the force from the walls.
Gothic cathedrals couldn't reach their dramatic heights without arcboutants—these external arches literally invented the way we could build impossibly tall stone buildings, allowing architects to push walls higher than ever before while keeping them from toppling over.
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