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=Opening in Wall= A window is an opening in the wall of a building that allows light to enter a room and people to see out. At previous times in history they were merely small oval or square holes in the walls.
Very early windows were shielded with hide or cloth stretched over the opening or wooden shutters. Later on two different types of windows were invented: mullioned glass windows, which multiple very small pieces of glass joined together with leading, and paper windows. Mullioned glass windows were the windows of choice among European riches, whereas paper windows were so economical and widely used in ancient China and Japan. In England glass only became common in the windows of ordinary homes in the early 17th century. Modern-style floor-to-ceiling windows only became possible after the industrial glass-making process was perfected.
Today a window can be made in any shape and size desired.
Modern windows come in many styles. These include:
The terms "single-light" or "double-light" (or "'more'-light") refer to the number of glass panes in a window.
see also: The window cleaner, Windex.
=Astronomical Window= In astronomy, an atmosphere can have a window for portions of the electromagnetic spectrum; that is, those wavelengths which pass through the atmosphere are said to "pass through a window".
=World War II= Window was the WWII UK codename for a system intended to confuse German radar. It consisted of huge volumes of aluminium foil strips cut to a length corresponding to the radar wavelength which were dropped from aircraft so producing huge numbers of spurious echoes. A modern corresponding technique is "chaff". Other radar confusing techniques included Mandrel, Piperack and Jostle.