Definition: A window sash is the framed part of the window which holds the sheets of glass in place. Most commonly, though, window sash refers to that part of the window which moves. Window sashes are typically found in the double-hung window style, in which one sash is positioned above a lower sash. The lower window sash has the capacity to slide vertically until it is nearly parallel with the upper sash. However, any type of window which moves (such as the casement window) can also be said to have sashes.