A sliding sash window or box sash window is made of one or more movable glazed panels, or "sash's which are often separated from other panes by glazing bars or muntins. Although any window with this style of glazing is technically a sash, the term "sash" is used almost exclusively to refer to windows where the glazed panels are opened by sliding vertically, (Box or sliding sash) or horizontally (Yorkshire sash) the Yorkshire sash did not work with the use of any weighting system, they simply slide sideways and almost exclusively, one side is fixed in place.

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The sliding sash, or box sash window operates vertically with the aid of counter-balance weights, generally cast iron and sometimes lead, which are concealed in a box, hence "Box sash window".

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The earliest vertically sliding sash windows had a fixed top sash; the lower sash slid upwards in a groove and was either wedged in position or held by pegs inserted into holes drilled in the frame. They were probably introduced from France sometime in the mid-I7th century. The double hung sash window, with a counter-weighting mechanism, appears to have been a British invention. This was an ingenious technological breakthrough that enabled a far more subtle and sophisticated system of ventilation to be achieved than was possible with the old, side hung casement. It used a system of hidden, counterbalanced weights to allow both top and bottom sash frames to be moved independently. The earliest surviving double-hung sash appears to date from I70I, however, by I720 double-hung sashes had spread as far as Holland and the British and Dutch colonies.

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As a precautionary measure against the spread of fire, the 1709 Building Act stipulated that the corners of a Box sash frame must be hidden behind the face of the brick or stone and that "no door or window frame of wood shall be set nearer to the outside face of the wall than four inches". In 1774 this distance was increased to nine inches, and nearly the entire frame had to be hidden behind the face of the wall. While this legislation was only applicable to the cities of London and Westminster, the styles they produced became fashionable and sliding sash window installations spread throughout England within about twenty years.

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While sliding sash windows of the late 17th century could be quite large, in the early 18th century they were relatively small, sometimes with a curved top, with thick glazing bars and small panes of glass. By 1730 brick arched windows had largely been replaced by square-headed varieties that were cheaper to make. The sliding sash glazing patterns inserted into these frames often took the form of six panes over six, although this was by no means the rule. Nor were the dimensions of each pane necessarily dependent on the principle of the golden section (a system of proportion used by the ancient Greeks and rediscovered during the Renaissance). In some cases, individual panes were broader than they were tall. The overall size of the box sash window was, nevertheless, usually kept in proportional harmony with the rest of the facade.

Sliding sash windows offer an elegance rarely matched by any other design , and we still see them being fitted in new properties today, and long may that be the case.

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