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What Is a Twin Mulled Window?

Gail Sessoms

Mulling is a window industry term that describes a type of window assembly. Windows assembled to attach one on top of the other are called “stacked” windows. Mulled windows are manufactured to assemble side-by-side. Mulling also includes more complex window constructions, including stacked and mulled window combinations and large window combinations that are assembled at the installation site. Simple window assemblies are mulled as twins, triples and quads.

Twin Mulled Window

Mulled windows are attached side-by-side.

The twin mulled window is two windows attached side-by-side. The triple and quad mulled windows include three and four attached windows, respectively. The traditional twin mulled window style requires installation of two windows and a mull bar. The mull bar, which is also called a mullion, is used with sealant to provide an air-tight connection between the side-by-side windows and the window frame.

Single Twin

Another twin mulled style window is a single window that appears to be two windows. The single style twin is a single-hung window, which means the entire window is hung inside of a single frame instead of separate frames for each window. The single window style twin mulled is installed as one unit.


Some mulled windows combine stacked, mulled and different types of windows in the same frame. One mulled window style combines one large window, usually a stationary picture window, with two smaller stacked windows at the side, top or bottom. One of the smaller windows can be opened by sliding it to one side.

Bay and Bow Windows

Bay and bow windows, which are assembled on site, combine stationary or fixed windows with windows that can be opened and closed. Bay windows feature a large picture window flanked by operable windows. A bow window usually combines stationary windows at the center with operable windows at the ends. Depending on the construction and the number of windows in the design, a bay window could include twin mulled windows at the sides. A bow window could include twin mulled windows at the center or sides.