A horizontal beam or stone that spans across an opening in a wall, particularly one that supports the weight above a doorway or window.
From 'breast' (the front part) and 'summer' (from Old French somier, meaning a pack animal or beam that carries load). Medieval builders used this term for structural support elements in walls.
Medieval masons called these beams 'summers' because they literally carried (or 'summed') up all the weight of the masonry above them—basically the 'pack animals' of castle architecture!
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