What are Window Mullions?

Window mullions are vertical pieces that separate sections of glass on a window. While modern versions are often decorative, for years, manufacturers used them to connect smaller pieces of glass together.

Today’s manufacturing process allows for large windows, making mullions less popular. Some homeowners still use mullions to connect windows or simulated mullions to add detail to expansive glass.

If you’re considering installing windows with mullions, here’s what you need to know.

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

Window Mullions

While similar sounding, window muntins and window mullions are different. 

Window mullions are vertical bars that combine two windows or divide a large window into sections. Before the Victorian era, window makers used mullions to connect smaller pieces of glass to construct one large window.

Today the primary purpose of mullion kits is to join two windows. Some homeowners also add simulated mullions to divide a large window into sections.

A window muntin is a vertical divider, often looking like a grid. In the past, muntins supported the glass in windows. Even though most windows no longer need muntins for support, many homeowners add them for looks. 

Today, you can order windows with muntins between the panes or buy snap-on versions that go inside the window.

What Do Window Mullions Look Like?

Most homeowners get window mullions and muntins mixed up. Window mullions are strips that join two windows together or make a large window appear divided. Window muntins look like decorative grids. 

Here are some examples of window mullions:

Window with Black Mullions

Window with Black Mullions
Hugh Jefferson Randolph Architects

The vertical black strips that go down this large window are mullions. They separate the window into sections, breaking up its look.

Kitchen Window with White Mullions

Kitchen Window with White Mullions

The area over the sink has a mullion strip in the middle, joining the two windows together. The windows on either side feature grid-like window muntins.

Contemporary House with Window Mullions

Contemporary House with Window Mullions
@Castanes Architects PS

Mullions work well for contemporary homes that feature expansive windows. On this home, the mullions divide each large window into three sections.

Black and White Window with Mullions and Muntins

Black and White Window with Mullions and Muntins

The white trim piece between these two windows is a mullion. The black grids on the top half of the window are muntins.

Frequently Asked Questions (FAQ)FAQ

Are window mullions outdated?

Window mullions aren’t outdated. You can use a window mullion kit to connect two windows or use simulated window mullions to split a large window into smaller sections.

Can you paint window mullions?

You can paint window mullions depending on their material. For instance, it’s much easier to paint a wood window mullion than a vinyl one.

Where can I buy window mullion kits?

You can buy a window mullions kit from Lowes or the Home Depot. You may also be able to purchase one from your window manufacturer.

Can you buy snap-in window mullions?

You can buy snap-in window muntins or grilles that give your window a grid look. But since mullions are one vertical piece and not as decorative, they may be harder to find as a snap-on version.

Final Thoughts

Window mullions join two windows or make a large window appear divided into vertical sections. While mullions aren’t as common as they were 100 years ago, they still prove practical for some purposes and can add interest to a home.

But, it’s crucial to note window mullions and muntins are different. So, if you’re looking for grid-like dividers, search for muntins. If you want to combine two windows, look for mullions.