What is the difference between primary and secondary glazing?

What is the difference between primary and secondary glazing?

Double glazed windows use two sheets of glass with a gap between them that creates an insulating barrier. The gap can either be filled with a heavy inert gas such as Argon or it can be a vacuum; both will act to slow the movement of heat from one side of the window to the other.

Secondary glazing involves installing retrofit supplementary glazing to the inside of an existing single-glazed window leaving an air gap between them. It can be a Perspex material, or a very cheap DIY install is made of plastic heat shrink film. Both forms can be very effective if installed correctly.