WHAT IS A SASH WINDOW
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. 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. The design of the sash window is attributed to the English scientist and inventor, Robert Hooke. The oldest surviving examples of sash windows were installed in England in the 1670s.
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. This type of window provides a maximum face opening for ventilation of one-half the total window area.
A significant advantage of sash windows is that they provide efficient cooling of interiors during warm weather. Opening both the top and bottom of a sash window by equal amounts allows warm air at the top of the room to escape, thus drawing relatively cool air from outside into the room through the bottom opening.