The Mullion in Architecture and Construction

old fashioned church style window with elaborate window pane shapes
  • Intermediate

Introduced even before the 10th century in Armenian, Saxon, and Islamic architecture, stone mullions were introduced with the invention of tracery. The mullion is by definition any vertical structural member dividing adjacent windows or lites.

It is slender in concept and usually backset within the enclosing structure of a facade enriched with carvings and moldings running unbroken around entire groups of windows.

What Are Mullions Exactly?

The mullions are part of a framework dividing it into separate sections. When you're looking at a group of windows placed next to each other, the vertical members separating each window or regrouping several windows within a larger cluster of windows further subdividing the array are called mullions.

But if a window is also constructed to support additional elements vertically as transoms or curtain windows, the horizontal bars separating the windows are also known as mullions.

When first introduced their main purpose was to help support the structure above and around it but they are also used, either vertical or horizontal, to divide and support smaller panes of glass when used to break up a space in a larger window.

The mullions in traceried windows usually have their edges shaped into chamfers, bevels, or more complex profiles and carvings, sometimes even further decorated with colonnettes.

The more dramatic effect is achieved with the slender mullion being backset from the face of the wall to let a series of molding run straight through across and around the whole group and subdivided groups of windows.

gray minimalist room with large french doors and curved window divided by muntins

How Have Mullions Evolved?

In more recent times, you see mullions made from a variety of materials such as stone, wood, metal, and plaster. They may not always act so much as structural components but can sometimes be meant more for a decorative appeal.

On a detailed frame, the mullions separate panes of glass, panels, or a door and the glass above or at the side, or sidelights. On a double door, it can separate the two sashes from each other and can be either fixed or removable by need, depending on its application.

The mullions between double doors offer greater security by providing greater rigidity at the center of the framework, and when made to be removable, they provide added width to the door opening in order to allow larger material or equipment to be brought in.

Where Do Muntins Fit In?

window with lace curtains and muntin frames letting sunlight in

Muntins are often confused with mullions. Muntins are only used as a decorative finished "en applique" molding where the mullions serve just as well as decorative as structural elements.

Both components can be either vertical or horizontal, both can look similar in appearance.

The mullions, however, as described earlier are actually inserted within the frame and physically separate different pieces or panes of glass from each other, and are used as support members to secure each piece of glass with clips, glazing points, and glazing putty, or other glazing trims.

Muntins are completely different altogether, both in design and purpose. Where mullions are visible on both sides of the glass or the frame, a mullion trim is only used on glass panes and is the section that sits on the glass surface, usually secured to the glass with a good quality two-sided tape.

Since both sides of a mullion have a totally different look or profile, so are the two sections of the muntin bars—the exterior part is presented with beveled cuts while the inside present a more fancy profile, usually a standardized quarter round on both edges.

Where the mullions are concerned, if a glass gets broken, only the affected lite will need replacement. With a window that has muntin bars, if the glass gets broken, the whole window and all most muntins will need to be removed and replaced.