A window is an opening in a wall, door, roof or vehicle that allows the passage of light and, if not closed or sealed, air and sound. Modern windows are usually glazed or covered in some other transparent or translucent material. Windows are held in place by frames. Many glazed windows may be opened, to allow ventilation, or closed, to exclude inclement weather. Windows often have a latch or similar mechanism to lock the window shut. Types include the eyebrow window, fixed windows, single-hung and double-hung sash windows, horizontal sliding sash windows, casement windows, awning windows, hopper windows, tilt and slide windows (often door-sized), tilt and turn windows, transom windows, sidelight windows, jalousie or louvered windows, clerestory windows, skylights, roof windows, roof lanterns, bay windows, oriel windows, thermal, or Diocletian, windows, picture windows, emergency exit windows, stained glass windows, French windows, and double- and triple paned windows. The Romans were the first known to use glass for windows, a technology likely first produced in Roman Egypt, in Alexandria ca. 100 AD. 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. In the 19th century American west, greased paper windows came to be used by itinerant groups. Modern-style floor-to-ceiling windows became possible only after the industrial plate glass making processes were perfected.

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