A sash window is a sliding window with one or two sashes that slide vertically in a frame.
English sash windows balance the lower, operable sash with wires and lead weights in the hollowed jambs so that the lower sash can be lifted and lowered with ease. Compared with side-hung windows, sash windows have the disadvantage of weak lateral and central seals.
The sash window was developed in France around 1550 and improved with a system of weights by the architect Robert Hooke in England in the 1660s.