Meaning of box sill

box' sill"

Pronunciation: [key]
— Carpentry. Carpentry.
  1. 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. Cf. L sill.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.