Sill

1.The lower horizontal part of a window frame or window opening; also the accessory member which extends as a weather barrier from frame to outside face of wall. 2. The horizontal member at the bottom of a window or door.

A submerged structure across a river to control the water level upstream. The crest of a spillway. The horizontal gate seating, made of wood, stone, concrete, or metal at the invert of any opening or gap in a structure. Hence, the expressions: gate sill, stoplog sill.