A sash window is one that is installed with at least one movable panel, popularly known as a sash. The sashes used form a frame whose function is to hold the glass panes often separated from other panes on the same window.
The number of glass panes used depend on the age or era of the sash window; some popular types include the Edwardian, Victorian, and Georgian eras.
These kinds of windows do not open on a hinge. Instead, sashes are installed in a way that they vertically lean on each other, one behind and the other at the front. Through lead counterweights, the formed frame or sashes slide side to side when opening or closing the window.