In the broadest definition, a window sill is a horizontal structural feature underneath a window opening or window unit in masonry or framed building that is regraded as part of the window frame. The bottom of a window frame rests on top of the wall opening's window sill. A hinged door or panel opens from the bottom up, so it always swings outward. The word "sill" comes from old English and means "a flat surface used for standing upon". In buildings, the sill serves as a base for walls within and provides stability for the building itself. In windows, the sill is the floor of the opening itself. It can be made of wood, metal, or concrete, but it must be able to support the weight of people or objects that might fall through the opening. The term "window sill" is used mostly for those that are made of wood. They are usually constructed out of two by four lumber that has had its sharp edges rounded off. There are also plastic sills available for the expensive luxury homes. These are generally made in one piece and are easy to clean. Wooden sills require regular cleaning to prevent insects from nesting under them. Stone sills are used instead. They are made of stacked rocks of various sizes. The larger the rock, the more stable it will be when supporting weight. Smaller rocks are used for visual appeal only since they cannot provide much support.