box sill

noun Carpentry.

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.

Compare L sill.

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018