The horizontal piece at the bottom of a door or window frame; the bottom part of a window opening; in geology, a horizontal intrusion of igneous rock.
From Old English 'syll', from Proto-Germanic 'suljo' meaning foundation or base. Related to Old Norse 'syll' and German 'Schwelle'. The geological meaning developed in the 19th century by analogy.
This humble architectural element carries deep symbolic weight - we sit on windowsills to contemplate, step over thresholds to enter new spaces, and speak of 'sill plates' as the foundation of buildings, making it a literal and metaphorical boundary.
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