earliest form of movable window, wood or metal framed, with hinges or pivots at the upright side of the vertically hung sash, so that it opens outward or inward along its entire length in the manner of a door. One frame, separately movable, of such a window is called a casement sash.

The British and German custom was to have the windows opening outward. A medieval English example exists at the Falstaff Inn, Canterbury, Kent, Eng., with casement windows below fixed windows, or lights, all composed of small leaded panes. The French casement commonly has two meeting leaves that open inward, requiring careful craftsmanship to prevent weather from penetrating them. These French casements were adapted in the United States chiefly as ways to give access onto balconies and porches, and in this doorlike form they are known as French windows, French doors, or casement doors.