What is a Mullion? (with photos)

The University of Cambridge’s King’s College Chapel is an example of a building with barred windows.

A mullion is an architectural device primarily used in glass windows, doors, and curtain walls. Mullions are typically used as vertical supports, but some horizontal frame members can 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.

Modern door mullions are windows made of wood, aluminum, steel or fiberglass.

The classic window frame is made of stone and was an important part of Gothic architecture. It was often found in churches and stained glass windows. Today, a stone mullion is most commonly found in patios and other outdoor spaces. The middle mullion splits an outer wall to allow air to pass through and does not contain any glass. Modern door mullions are windows made of wood, aluminum, steel or fiberglass.

Traditionally, mullions were used as structural support for other building elements. They helped frame the window frame and provide additional stability for opening a door or window. These structural elements were also used to break large windows so that smaller panes of glass could be used, as larger sheets of glass were prohibitively expensive until the late 20th century. Today, these mullions primarily serve as a decorative feature and can be used to complement a variety of decorations 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 can be square, rounded or intricately shaped like wood molding. Depending on the material chosen, you can paint, stain or wax a mullion to protect it from the weather and create the desired finish.

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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 can be attached to the surrounding structure or left uncut, although this technique is often dictated by wind levels and standards defined by local building codes. When two windows are placed side by side without a separate casement, the two joined pieces of the casement are often called a mullion. The thinner vertical frame pieces that separate a window frame into individual panels are generally known as transoms, or bars of glass, rather than mullions.

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

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