An enclosed space containing the stairs in a building, usually accessed from different floors.
From 'stair' (Old English stægr, a step or passage) plus 'well' (Old English wella, a spring—used for enclosed spaces). Combined in modern English to name the staircase structure.
Stairwells are actually scientifically interesting—they're used 8x less than elevators even in low buildings, partly because people don't see them until they're looking for an exit (they're literally invisible in our perception).
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