window, in architecture, the casement or sash, fitted with glass, which closes an opening in the wall of a structure without excluding light and air. It may have a square, round, or pointed head; may be single, double, or grouped; in relation to the wall, it may be flush, recessed, or projected. A projected window is called a bay window if polygonal, a bow window if semicircular, an oriel if it has corbeled brick or stone supports. A mullioned window is divided by slender bars into panes; when the bars radiate from the center of a circular bar it is called a wheel. It takes the name of rose window when adorned with stained glass or figure design. The long, narrow window of the English Perpendicular Gothic church is called a lancet; a lunette fills a somewhat crescent-shaped space under a vaulted intersection high upon a wall. A fanlight, characteristic of the American Colonial style, is either a semicircular transom, usually over an entrance, or a small attic window (or often a pair flanking the chimney). A French window reaches the floor and has double casements opening as doors; originating in France in the late Renaissance, it was adopted throughout the Continent and in the Southern states in America. The double-hung sashes (sliding up and down within the frame), first used in Renaissance England, attained wide popularity. In Spain windows are frequently ornate, with stone framework, an elaborate head, and a decorative iron grille. In Indian and Byzantine windows a pierced slab of marble or alabaster often substitutes for glass. Muslims also used cement frames in which colored glass was set in brilliant arabesque forms. Carved and turned wood grilles are found in Syria and Egypt. In China and Japan, rice paper, protected by a sliding wooden shutter, often takes the place of glass. Shell, also used in China, was employed by the Romans, as were thin panes of marble, mica, and horn. In modern architecture the use of windows has greatly increased in dwellings and in the exterior walls of factories and commercial buildings.