Transom windows were named because of their placement. They are located just above the transom, the solid beam that separates the top of a door or window from the rest of the wall. The windows admit light and air into the building.
Transom windows are usually long, narrow rectangles sized to be compatible with the door or window. Other shapes are also possible. Semi-circular, or fan-shaped windows were once especially popular above front doors. In some areas, windows with this shape were called fanlights.
Before electric lighting, building interiors were often quite dark, even in the middle of the day. Placing transom windows above interior doors allowed light to penetrate further into the building. They are regaining popularity today as more homeowners prefer natural, rather than artificial, light in their homes. Homeowners also appreciate the security value of these windows, which let in light even while the door below remains securely locked.
Some transom windows are fixed and cannot be opened. These windows are often decorative in some way. They may be divided into multiple panes by vertical muntins, or they might be filled with stained glass. The decorative value of these windows was greatly appreciated in the Victorian period.
Other transom windows can open. Sometimes the hinges are placed at the bottom of the window, so the window opens at the top. In this arrangement, a lightweight chain is often attached so that the window cannot fall too far. These windows can also be hinged at the top, so that they open at the bottom.
Air circulation is greatly improved with the installation of transom windows that open. When the room's exterior windows are opened and the transom window is also open, fresh air is drawn into the room and hot air is expelled through the window. This method of creating air movement made many Victorian homes comfortable even in hot weather. Today, the use of a house fan makes the system even more efficient.