box sill
- a sill for a building frame, composed of a plate resting on the basement wall and a joist or header at the outer edge of the plate, as well as a soleplate for the studs resting either directly on the joists or on the rough flooring.
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Compare L sill.
Dictionary.com Unabridged
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018