What is a Mullion Window?

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

A window is more than just glass. In this lesson, we are going to check out mullions and see how they were first used, as well as how their appearance and use changed over time.

The Mullion

Have you ever been trying to look out a window, but your view was partially blocked by a thick, vertical beam of some sort? It's frustrating when this happens, but even more frustrating when you don't know what to call the stupid thing. Well, it's called a mullion. In window design, a mullion is a vertical divider that breaks apart the opening. Sometimes, thin horizontal dividers within the window are also called by this name. Regardless, any window with these features is said to be ''mullioned''. Mullioned windows are common in practically all forms of Western architecture (and many forms of Islamic architecture as well), so you're likely to run into them again. At least now you know what to call them.

Those thick, vertical elements are known as mullions

History and Function

Understanding the mullion's use requires an understanding of its history. They first appeared interspersed across the medieval world, as people in Europe and around the Mediterranean tried to find new ways to let natural air and light into a structure. If you don't already know how to build sturdy windows, especially in large buildings, it can be hard. After all, every hole in the wall is a part of the structure that is not supporting weight.

The first medieval style to really rely on mullions was the Romanesque, which was popular around the 11th century. Romanesque buildings were large, imposing, and sturdy stone structures inspired by the grandeur of ancient Rome. Architects tried to create larger interior spaces, and found ways to places numerous open-air windows in the walls. To support these windows, a thick stone pillar was added, and the mullion as we know it was born. The purpose was to support the arch or lintel above the window, which was bearing the weight of the heavy stone wall. Mullions let Romanesque architects line their structures with openings, improving light and airflow in otherwise thick and stuffy buildings.

Early mullioned windows did not tend to be very large

The Romanesque style was replaced by the Gothic, in which the size and number of windows exploded. This was also the first time that glass was largely available. By the Gothic era, architects knew more about structural stability and the mullion was not quite as important in maintaining the stability of the window opening. It did, however, have a very important role in supporting the glass. While Gothic architects were able to produce and use glass, it was generally only in small panes. Thus, Gothic windows often needed more than one mullion. Multiple mullions helped provide structural support to the numerous panes of glass that made up the big, Gothic windows.

Of course, with these windows being such visible features, it was important for the mullions to look good as well. Gothic mullions were structural as well as decorative, arranged in unique and ornamental patterns that accentuated the design of the window.

Most mullions are decorative, as well as structural

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