A Sill refers to the bottom, horizontal part of a window frame which provides the necessary support for the window frame as well as being a decorative element. It may also be used to refer to the bottom of a door frame, though this is more commonly called a threshold. An exterior window sill channels water away from the window and the building as water can rot wood and cause brick to crumble. Sills are usually constructed out of the same material as the rest of the window frame, which may be wood, metal or vinyl. In brick construction, an additional brick sill tends to be added to the exterior of the building for design purposes.

WIKIPEDIA ENTRY FOR WINDOW.