Comparing Window Muntins and Mullions

Are you planning a window replacement project? As you approach various products and contractors, you’ll sometimes encounter unfamiliar terms such as “muntin” and “mullion.” Because these terms sound almost alike, many people confuse their definitions with each other. Kroll Construction explains the differences here:

Window Muntins and Mullions


The Oxford English Dictionary defines a mullion as the vertical bar between the glass panes of a window. Compared to muntins, mullions are thicker and heavier in weight because they’re traditionally designed to divide larger window frames while keeping them in place. Mullions do not refer to both the vertical and the horizontal pieces, but only to the vertical pieces of wood separating the panes of glass. 


Muntins, on the other hand, technically refer to any kind of vertical divider on wood panels, furniture, doors and so on. These are the vertical dividers that separate glass panes in windows. They were traditionally designed to provide additional support to the outer walls of early structures that had large windows. This structural support prevented the weight of the walls from breaking the large glass and collapsing, by keeping the weight transmitted vertically.

How Can You Tell Them Apart?

Many window companies don’t normally prioritize differentiating the two terms because not all buyers take the time to know their definitions. They also don’t typically affect a modern window’s performance as they’re commonly made by sandwiching thin strips of aluminum or plastic between a double-paned glass. However, understanding the difference may be necessary if you’re planning to add either mullions or muntins as a visual upgrade for your new windows. Both can also be considered window grilles, which is a more universal word that covers mullions and muntins as well as other relative terms such as windowpane dividers and grids.

Make sure to call Kroll Construction if you’re considering a major upgrade for your next exterior home improvement project. Reach us at (888) 338-6340 or you can fill out our convenient online contact form. We serve residents of Michigan, including Detroit and the surrounding communities.

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