Also known as window bottom or window ledge, a window sill is a flat, shelf-like piece of window trim that is placed at the window base. Not only do window sills improve the overall functionality of the window, but they also upgrade the design of the element. Black Granite window sills are no longer drab, possibly out-of-place sills; rather, they are now a focal point that accentuates the overall value of the project. Window sills are an important part of a property’s structure and they keep the window frame in place.