The lowest member of the frame of a structure, resting on the foundation and supporting the floor joists or the uprights of the wall (also sill plate or mud sill). Also, the member forming the lower side of an opening, as a door sill, window sill, etc.
The lowest member of a wood frame structure that rests on the foundation. It supports the floor joists or uprights of the wall. It can also refer to any member forming the lower portion of an opening, such as a door sill or window sill.