Window

A window is an opening in an otherwise solid, opaque surface through which light and sometimes air can pass. For example, a window can be in the wall of a house, or in a scientific measurement device that permits reading of certain wavelengths.

Window is also used metaphorically to refer to a time period during which something can occur and outside of which the thing cannot occur, as in a window of opportunity to launch a rocket to the moon on the most efficient trajectory.

The word Window dates back to Old Norse "Wind Eye"; opening to the air.

Opening in wall

A window is an opening in the wall of a building that allows light and air 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 with wooden shutters. Later, various types of windows were invented that allowed light but not weather to pass into a building: mullioned glass windows, which joined multiple small pieces of glass with leading, paper windows, flattened pieces of translucent animal horn, and plates of thinly sliced marble. Mullioned glass windows were the windows of choice among European well-to-do, whereas paper windows were economical and widely used in ancient China 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 Northern Britain. Modern-style floor-to-ceiling windows became possible only after the industrial glass-making process was perfected. Evidence of glass window panes in Italy dates back nearly 3000 years.

Modern windows are customarily large rectangles or squares with glass surfaces. Churches traditionally have stained glass windows.

Today a window can be made in practically any shape and size desired.

Window styles

Modern domestic windows come in many styles. The choice of design varies throughout the world, and is largely dictated by the prevailing weather conditions. Coastal climates tend to have smaller outward-opening windows due to the stronger winds experienced - e.g. England. Continental climates tend to have larger windows, many of which open inwards - e.g. France and Germany. Styles available include:

Double-hung sash window: The traditional style of window in the USA, and many other places that were formerly colonized by the UK, with two parts (sashes) that overlap slightly and slide up and down inside the frame. The two parts are not necessarily the same size. Nowadays, most new double-hung sash windows use spring balances to support the sashes, but traditionally, counterweights held in boxes either side of the window were used. These were and are attached to the sashes using pulleys of either braided cord or, later, purpose-made chain. Double-hung sash windows were traditionally often fitted with shutters. Sash windows may be fitted with simplex hinges which allow the window to be locked into hinges on one side, while the rope on the other site is detached, allowing the window to be opened for escape or cleaning.

A stained glass panel depicting various Biblical scenes at a historic church in Scotland.

Single-hung sash window: one sash is movable and the other fixed. This is the earlier form of sliding sash window, and is obviously also cheaper.
Horizontal Sliding sash window: has two or more sashes that overlap slightly but slide horizontally within the frame. In the UK, these are sometimes called Yorkshire sash windows, presumably because of their traditional use in that county.
Casement window: An outward-opening window comprising either a side-hung, top-hung, or occasionally bottom-hung sash or a combination of these types, sometimes with fixed panels on one or more sides of the sash. In the USA these are usually opened using a crank, but in Europe they tend to use projection friction stays and espagnolette locking. Formerly, plain hinges were used with a casement stay.
Tilt and slide: a window (more usually a door-sized window)where the sash tilts inwards at the top and then slides horizontally behind the fixed pane.
Tilt and turn: a window which can either tilt inwards at the top, or can open inwards hinged at the side.
Jalousie window: A window comprised of many slats of glass that open and close like a Venetian blind, usually using a crank. A Jalousie door is a door with a Jalousie window.
Clerestory window: A vertical window set in a roof structure or high in a wall, used for daylighting.
Skylight: A flat or sloped window built into a roof structure for daylighting
Bay window: A multipanel window, with at least three panels set at different angles to create a protrusion from the wall line.

Oriel window
Fixed window: A window that cannot be opened, whose function is limited to allowing light to enter. Clerestory windows are often fixed.
Picture window: A very large fixed window in a wall, intended to provide an unimpeded view as if framing a picture.
Divided-light window: A window made up of smaller panes that are held together by muntins. New windows are often a single pane with faux muntins glued on to give the appearance of an old dividing-light window. This is cheaper and more energy efficient.
Emergency exit window or egress window: A window big enough and low enough so that occupants can escape from the openable part in an emergency, such as a fire. In the United States, exact specifications for emergency windows in bedrooms are given in many building codes. Vehicles, such as buses and airplanes, sometimes have emergency exit windows as well.

Technical terms

Etymologically speaking, any window can be called a "light". However, within the window industry, particularly in insulated glass production, the term "lite" (so-spelled to keep the meaning differentiated from actual sunlight) is used to mean a single glass pane, several of which may be used to construct the final window product. For example, a sash unit, consisting of at least one sliding glass component, is typically composed of two lites, while a fixed window is composed of one lite. The terms "single-light", "double-light" etc refer to the number of these glass panes in a window.

In the USA, the term replacement window means a framed window designed to slip inside the original window frame from the inside after the old sashes are removed. In Europe, however, it usually means a complete window including a replacement outer frame.

The USA term new construction window means a window with a nailing fin designed to be inserted into a rough opening from the outside before applying siding and inside trim.