A window with two panes of glass is crucial for energy efficiency. The unit in which the two pieces of glass are held is called an IGU, or an insulated glass unit. This unit contains the pane of Low-E glass, a spacer system and argon gas. The level of efficiency a window or door can reach is determined by the investment in the IGU, which is the key to a high performance window. There are many levels of investment that a window manufacturer can put into their IGU, and consequently varying levels of performance. For reference, a monolithic window, which consists of a single sheet of glass, does not have an IGU.