A "sill" is a horizontal structure that forms the bottom part of a window or door frame. It serves as a support for the frame and helps to keep out water and drafts. Sills can be made from various materials, including wood, stone, or metal, and they often extend slightly beyond the frame to provide additional protection. In addition to windows and doors, sills can also refer to the ledges found at the base of other architectural features, such as fireplaces or cabinets. They can be decorative or functional, providing a space for plants, decorations, or other items.