A cantilever, in construction, is when a horizontal beam extends beyond a foundation wall. The beam is only attached at one end. The beam is stabilized by the weight of the wall. The part that extends freely is called the cantilever. A bay window is a three-sided window that extends beyond the exterior wall of a house or building. The center window runs parallel to the wall and the side windows may box in or angle back, depending on the bay style. This window is called a bay window because from inside the room it forms the shape of a bay.

Bay windows have been built for more than 500 years. Often the popularity of this window increases in dense housing conditions where the costs of land increase and housing is forced upward in order to gain interior space. In this way, the bay window can project beyond the outer wall providing both a shelter below, additional interior space and additional light for that interior space.

Bay windows come in many styles; the window style that is sometimes referred to as a box window today is simply a boxlike version of a bay window. When a bay window has curved windows, it is called a bow window. If the exterior support of the bay is made of corbelled brick or stone, it is called an oriel. Most bay windows have three windows, but some have as many as five or more.

There are two common types of bay windows as well as a window often called a bay window that is actually a noncantilevering polygonal-shaped room with exterior windows. This third style of window is configured directly above the foundation of the building or house. It rises from the foundation and it is part of the exterior wall even though it creates a baylike space on the interior of the building.

The two real bay windows both cantilever beyond the exterior wall. One style cantilevers just below the bottom of the windowsill. The other style cantilevers at the floor of the room that has the bay window.

The bay window that cantilevers directly below the windowsill may suffer with support issues as the weight load of the window and window roof may rest on brackets. This style limits the depth of the bay window and may be somewhat less durable. From the inside of the room with this type of bay window, the window will project away from the exterior wall at or near two feet above the floor. No additional interior floor space is gained, although additional light is brought in. This style of window invites a window seat or room for decorations.

The bay window that cantilevers from the floor level of the room is stronger. Usually the flooring beams will extend beyond the exterior wall. This allows for considerably more load on the surface being projected beyond the wall. The resulting bay window can be somewhat larger and, because the floor beams are projecting outward, the interior floor space of the room increases. This is a substantially better design in terms of engineering stresses. It is also more aesthetically appealing on the outside of the building since the windows will not look cut off or squashed. When just a window projects outward, the glass may make the window area look weak or thin, and the roof of the bay may appear to push down visually on the glass. Floor level bay windows are more prevalent.