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What is a Mullion?

By B. Turner
Updated May 16, 2024
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A mullion is an architectural device primarily used in windows, doors, and glass curtain walls. Mullions are typically used as vertical supports, but some horizontal framing members may also be considered types of mullions. They are also used as part of a door frame to divide an opening into two separate sections, which allows a single frame to accommodate two separate doors.

The classic window mullion is made of stone, and was a major part of Gothic-style architecture. It was frequently found in churches and stained glass windows. Today, a stone mullion is most commonly found in courtyards and other open-air spaces. The mullion divides an exterior wall to allow air to pass through, and does not contain any glass. Modern mullions in doors are windows are made from wood, aluminum, steel, or fiberglass.

Traditionally, mullions were used as a structural support for other building elements. They helped frame the window jamb and provide additional stability for a door or window opening. These structural elements were also used to break up large windows so that smaller panes of glass could be used, as larger sheets of glass were prohibitively expensive until the end of the 20th century. Today these mullions primarily serve a decorative feature, and can be used to complement a variety of decors and architectural designs.

Builders can use many different types of mullions when creating windows or curtain walls. The mullion can be created using different profiles, and may be square, rounded, or intricately shaped like wood molding. Depending on the material that is chosen, one can paint, stain, or wax a mullion to protect it against the elements and create the desired finish.

There are also several ways to install these devices. They are often placed between two separate window units to act as a decorative accent. The ends of the mullion may be clipped to the surrounding structure or left unclipped, though this technique is often dictated by wind levels and standards set by local building codes. When two windows are placed side-by-side without a separate mullion, the two joined pieces of jamb are often referred to as a mullion. Thinner vertical framing pieces that separate a window sash into individual panes are usually known as muntins, or glazing bars, rather than mullions.

In sets of double doors, the vertical framing member between the doors is also known as a mullion. These mullions are often removable, which allows building owners to bring large objects in through the doorway. When not in use, these devices are locked using a traditional cylinder and key.

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