Window, building openings for air and light. Some windows open inward or outward from the opening, and these are called casement or hinged windows. Many windows are double-hung and are opened by lifting them up or pushing them down, and these are called double-hung windows. Double-hung windows were a Dutch invention (1800s). The arrangement of windows—known as fenestration—in a building is an important part of its overall design. Stained-glass windows, in which colored bits of glass within metal frames create a picture or pattern, were an important part of church design from the Middle Ages.