Double-paned windows have two parallel panes (slabs of glass) with a separation of typically about 1 cm; this space is permanently sealed and filled at the time of manufacture with dry air or other dry nonreactive gas. Such windows provide a marked improvement in thermal insulation (and usually in acoustic insulation as well) and are resistant to fogging and frosting caused by temperature differential. They are widely used for residential and commercial construction in intemperate climates. Triple-paned windows have been commercially manufactured and marketed with claims of additional benefit but have not become common. In the UK double-paned and triple-paned are referred to as double-glazing and triple-glazing.
Over the centuries techniques were developed to shear through one side of a blown glass cylinder and produce thinner rectangular window panes from the same amount of glass material. This gave rise to tall narrow windows, usually separated by a vertical support called a mullion. Mullioned glass windows were the windows of choice among European well-to-do, whereas paper windows were economical and widely used in ancient China, Korea and Japan. In England, glass became common in the windows of ordinary homes only in the early 17th century whereas windows made up of panes of flattened animal horn were used as early as the 14th century.
An awning window is a casement window that is hung horizontally, hinged on top, so that it swings outward like an awning. In addition to be used independently, they can be stacked, several in one opening, or combined with fixed glass. They are particularly useful for ventilation.
The English language-word window originates from the Old Norse 'vindauga', from 'vindr – wind' and 'auga – eye', i.e., wind eye. In Norwegian Nynorsk and Icelandic the Old Norse form has survived to this day (in Icelandic only as a less used word for a type of small open "window", not strictly a synonym for gluggi, the Icelandic word for window), in Swedish the word vindöga remains as a term for a hole through the roof of a hut, and in the Danish language 'vindue' and Norwegian Bokmål 'vindu', the direct link to 'eye' is lost, just like for 'window'. The Danish (but not the Bokmål) word is pronounced fairly similarly to window.