A window sill is part of the window trim; the decorative covering around the window-s edge that protects where the window attaches to the wall and prevents drafts and moisture from getting into the home. the window sill is the flat piece at the bottom of the window. the other parts are the head casing at the very top; the side jambs to the left and right of the window; and the apron positioned directly below the window sill. together; they offer complete protection around the window. views - sections . autocad - dwg.