Double Hung Window
Two sashes which, when both are closed, are positioned one immediately above the other. Both sashes can be slid up and down in tracks, but are kept from dropping to the sill by counterweights attached by cords or chains and concealed in the window casing.
By the nature of their design, double-hung windows can be opened to a maximum of 50% of the area of the window opening.
See also: Window.