A good way to understand sash windows is to understand a window sash. A window could be compared to a door. The door structure has a casement, and a door that is set on hinges within it. A window structure has a casement as well, but because it is made of glass and obviously more fragile, the pane or panes of glass must be held within a frame, called a sash. Whether it has hinges or slides within the casement is what decides what the style is.