Window is an opening in a wall or door to admit light and air into a closed space. The term may also refer to the glass placed over such an opening. One or more flat sheets of glass called panes cover most windows. Bars known as mullions hold the panes in place. The panes and mullions fit into a frame called a sash.
Some windows are designed to be permanently closed. There are two basic types of movable windows—hinged or casement windows and double-hung windows. Some hinged or casement windows have two sashes, one hinged to each side of the window. The sashes open inward or outward like a door. This was the first type of movable window. A double-hung window consists of two sashes hung one above the other. The lower sash slides upward on a track to let in air. The double-hung window was invented in the Netherlands about 1680 and rapidly became very popular.
Most windows are placed flat in a wall surface. However, there are also other designs. For example, dormer windows are cut through the roof of a building. Bay windows, bow windows, and oriel windows project out from a wall.
Until the 1800’s, only small panes of glass could be manufactured. The invention of improved machinery in the 1850’s permitted the production of large panes. This development had a major impact on the appearance and design of buildings. During the early and middle 1900’s, huge panes of glass, often mirrored or tinted, became popular for large buildings. They are too large and heavy to be opened. But advancements in heating and air conditioning have made such windows practical.
In early times, people who lived in warm climates left windows uncovered. In cold climates, people put animal skins over windows for protection from the weather. People in the Far East used paper to cover windows.
During the Middle Ages, craftworkers invented stained glass for use in church windows. A stained-glass window consists of pieces of colored glass arranged to form figures and decorative patterns. Metal bands hold the pieces in place. Stained-glass windows made in tall, thin, pointed shapes are called lancet windows. Stained-glass windows made in a circular shape are called rose windows.
