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 muntin bars.

Double-hung sash window
The traditional style of window in the USA, and many other places that were formerly colonized by the UK, with two parts (sashes) that overlap slightly and slide up and down inside the frame. The two parts are not necessarily the same size. Nowadays, most new double-hung sash windows use spring balances to support the sashes, but traditionally, counterweights held in boxes either side of the window were used. These were and are attached to the sashes using pulleys of either braided cord or, later, purpose-made chain. Double-hung sash windows were traditionally often fitted with shutters. Sash windows may be fitted with simplex hinges which allow the window to be locked into hinges on one side, while the rope on the other side is detached, allowing the window to be opened for escape or cleaning.

[edit] Single-hung sash window
One sash is movable (usually the bottom one) and the other fixed. This is the earlier form of sliding sash window, and is obviously also cheaper.

[edit] Horizontal sliding sash window
Has two or more sashes that overlap slightly but slide horizontally within the frame. In the UK, these are sometimes called Yorkshire sash windows, presumably because of their traditional use in that county.

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 muntin bars.[1] 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" or sliding sash. Sash windows are common in Europe and former colonies including the United States and many developing nations. The design of the sash window is attributed to the English scientist and inventor, Robert Hooke[2]

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, as the image illustrates.

Exceptionally wide sash window (approximately 2m wide) in a pub in Bromyard, HerefordshireTo facilitate operation, the weight of the glazed panel is usually balanced by a counter-weight concealed within the window frame. This is connected to the window by a sash cord or chain which 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 site is detached, allowing the window to be opened for escape or cleaning.

The name "hung sash window" is more usual in the United States, and typically refers to a double hung window with two sashes that can move up and down in the window frame. A single hung window has two sashes but normally the top sash is fixed and only the bottom sash slides. Triple and quadruple hung windows are used for tall openings, common in New England churches.

Construction is usually of softwood, and units are generally single glazed; although double-glazed sashes are available it is more common for single-glazed sash windows to be replaced with top-hung casements when double glazing is retro-fitted. Some top-hung double-glazed units are manufactured to give the appearance of sashes.

Traditional problems with wooden sash windows include rot, swelling or distortion of the woodwork, rattling in the wind (due to shrinkage of the wood), and problems brought on by careless application of paint. The sliding mechanism makes sash windows more vulnerable to these problems than traditional casement windows. Sash windows are relatively high maintenance, but offer advantages in return (looks, abides by laws (relating to older houses and buildings), natural resources etc). It is also possible to clean all the glass from within the building by sliding the two panes to different positions.