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The Difference Between Window Muntins and Mullions

There are many things that can affect your new windows’ appearance, including mullions and muntins. While these two appear similar, they’re actually two different things. The reliable window installation company in the area, Renewal by Andersen® of Rapid City, shares how you can tell them apart. 

The Difference Between Window Muntins and Mullions

What are Muntins?

They refer to the vertical dividers separating the glass panes in a window. Muntin only applies to the vertical pieces inside the window, however. The outer pieces forming the window frames are called stiles and rails instead. These days, muntins are applied to the windows for aesthetics – but it used to have a more practical purpose centuries ago. Crafting and transporting a large section of glass without breakage was nigh impossible then. Plus, the outer walls of buildings could not handle the weight of larger windows when they were installed. Muntins were introduced to provide structural support. By dividing the glass panes into smaller pieces and connecting them with muntins, they were able to create larger and more stable windows.

What are Mullions?

These refer to the vertical bars that divide two sides of a single window, creating the illusion of two windows when there’s actually only one. Like the muntins, mullions served as a necessary support structure to larger windows before the advent of lighter and less expensive plate and tempered glass. The use of mullions have fallen out of favor after World War II and if you still see them in newer windows, it’s only to serve as a stylistic device.

Today, muntin and mullion fall under the more convenient universal term: grille. The grille is a one-size-fits-all term used to describe anything that divides the panes of a window. It’s available in a wide array of lovely patterns. Our Renewal by Andersen windows, for example, can be customized with your choice of Colonial, Farmhouse, Prairie, and Modified Prairie grille patterns.

Schedule a Free Window Evaluation

Renewal by Andersen® of Rapid City can enhance the look of your new windows – and add to your home’s architectural accuracy – with our selection of grille patterns. Learn more about our options by calling us today at (605) 341-7831 or filling out this contact form. We proudly serve homeowners in Rapid City and other areas in Pennington County.


"The Andersen crew was very prompt and professional, and was finished in 1 day. Great!"

Terri Vaughn
May 10

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