"...Windows are generally one of three types: fixed, hung and casement. Fixed windows do not open, hung windows slide up or down, and casement windows open outwards. Windows were single-glazed until the 19th century, meaning that the panes of windows were a single thickness of glass. With industrialization came innovations in energy conservation, and window manufacturers began to use double-glazing, with the sealed space between the two layers of glass acting as insulation..."