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Window Mullions and Window Muntins: What’s the Difference?

Mullions and muntins are often used interchangeably, but it’s interesting to note that these two features are distinct from each other. Renewal by Andersen® of Montana, a top-rated provider of sliding windows, discusses the difference between muntins and mullions.

Window Mullions

Mullions are basically the vertical bars that you can find between the panes of glass in a window. They go way back the pre-Victorian Age when manufacturing large sheets of glass was still impossible. In order to create large expanses of windows, smaller panes of glass were attached to each other and held together by supportive mullions.

Window Muntins

Muntins are fairly common nowadays, but the term is often referred to in window replacement projects as any kind of vertical divider. Their main purpose in the early years, however, was to ensure that the weight of large windows were transmitted vertically on the outer walls of a building. The walls of old buildings usually could not support the weight of large windows, so muntins were considered structurally necessary.

Considering the main roles that muntins and mullions used to play, these features basically do not have any structural function nowadays. Homeowners simply add them to their windows for aesthetic reasons. That said, muntins and mullions are now generally lumped into one single category called “grilles.” Any window element that no longer has a structural function belongs to this group. Keep in mind, though, that adding any of these features to your windows will likely incur an extra charge. If you have a historic home and want to preserve its traditional look, muntins and mullions would surely be a good option to consider.

Renewal by Andersen of Montana offers an array of window styles including casement windows, double-hung windows and picture windows among others. Our team is composed of professional installers who will ensure quality results for your project. Call us at (406) 259-3944 or fill out our contact form to request a free estimate.


"I highly recommend Renewal by Andersen. We were treated well and everything went smoothly from start to finish. The company kept in contact with me before, during, and after the installation and let me know what to expect. I'm in love with my new windows! I will be using this company again when we refinish our basement. "

July, 21

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