A sash window or hung sash window is made of one or more movable panels or “sashes” that form a frame to hold panes of glass, which are often separated from other panes (or “lights”) by narrow muntins. Although any window with this style of glazing is technically “a sash”, the term is used almost exclusively to refer to windows where the glazed panels are opened by sliding vertically, or horizontally in a style known as a “Yorkshire light”, sliding sash, or sash and case (so called because the weights are concealed in a box case). The oldest surviving examples of sash windows were installed in England in the 1670s, for example at Ham House.[2][3] The invention of the sash window is sometimes credited, without conclusive evidence, to Robert Hooke.

The sash window is often found in Georgian and Victorian houses, and the classic arrangement has three panes across by two up on each of two sashes, giving a “six over six” panel window, although this is by no means a fixed rule. Innumerable late Victorian and Edwardian suburban houses were built in England using standard sash window units approximately 4 feet (1.2m) in width, but older, hand-made units could be of any size. Each sash is provided with springs, counterweights, or compliant weatherstripping to hold it in place in any location.

To facilitate operation, the weight of the glazed panel is usually balanced by a heavy steel, lead, or cast iron sash weight or counter-weight concealed within the window frame. The sash weight is connected to the window by a sash cord or chain that runs over a pulley at the top of the frame, although spring balances are sometimes used. Sash windows may be fitted with simplex hinges, which allow the window to be locked into hinges on one side while the counterbalance on the other side is detached, allowing the window to be opened for escape or cleaning.