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Understanding window terminology can feel confusing, but knowing the difference between mullions and muntins helps when choosing the right windows for your home. These architectural elements may sound similar, but they serve different purposes in window design.
What Are Window Muntins?
Muntins (also called grilles) are the vertical or horizontal strips that divide individual glass panes within a single window frame. Originally, these dividers served a crucial structural purpose when large sheets of glass were impossible to manufacture.
The term “muntin” traces back to 1688 in England, where the author R. Holme referenced “moontans and panels”. An 1850 English architectural glossary provided the clearest definition, describing muntins as “the intermediate upright bars of framing,” while calling the outer uprights “stiles”.
Early builders used muntins by necessity rather than aesthetics. Building walls couldn’t support the weight of large windows, so muntins allowed weight to be transmitted vertically throughout the structure. This was especially important in buildings with extensive windows, like Westminster Chapel, where the west window reaches nearly 45 feet high.
What Are Window Mullions?
Mullions are the thicker vertical bars that separate distinct window units or sections within a larger window frame. Think of them as the structural dividers between two separate windows placed side by side.
According to the Australian Glass and Window Association, mullions are “the vertical framing members between window units”. Unlike muntins, which divide glass within a single window, mullions divide entire window assemblies.
Before the Victorian era and affordable plate glass, mullions were essential structural elements. They supported smaller glass panes when manufacturing large sheets was prohibitively expensive or technically impossible.
Flying Mullions
A “flying mullion” is a movable mullion that separates an operable window panel from a stationary one. These slide or fold out of the way to create larger openings in multi-panel windows, commonly found in sunrooms, conservatories, or patio doors.
The Post-War Shift
After World War II, both mullions and muntins fell from favor. With tempered plate glass becoming available for residential use, smaller divided panes were no longer structurally necessary. These architectural elements became viewed as old-fashioned, obstructive, and unnecessary.
In 1956, architect Minoru Yamasaki (who later designed the World Trade Center) campaigned against window mullions for his Lambert-St. Louis International Airport design, calling them a “blunder.” By 1963, mullions were deliberately removed from a Chicago Lake Shore Drive building because they “cluttered the beautiful view of Lake Michigan.”
Modern Window Grille Trends for 2024-2025
Today’s window design landscape tells a different story. While mullions and muntins no longer serve structural purposes, they’ve experienced a stylish resurgence as decorative elements.
Current Design Preferences
Window grids are making a comeback with simplified, modern patterns rather than the intricate colonial-style grids of the past. Black-framed windows with larger, cleaner grid patterns have become particularly popular, creating sleek looks that work beautifully in farmhouse, industrial, and contemporary homes.
The 2025 trend emphasizes minimalism with geometric patterns, horizontal or vertical bars, and laser-cut designs that add visual interest without overwhelming the view. Homeowners increasingly choose larger glass panes with fewer, more strategic grille placements.
Modern Grille Construction
Contemporary windows use three main grille styles:
True Divided-Lite Muntins: Individual pieces of wood, vinyl, or aluminum assembled to create authentic grid patterns that separate actual glass panes.
Simulated Divided-Lite Muntins: Grilles attached to the exterior and interior glass surfaces, creating the divided-lite appearance without multiple separate glass pieces.
Between-the-Glass Muntins: Thin strips of aluminum or plastic sandwiched between double-paned glass units, offering the easiest maintenance since they never require cleaning.
Window manufacturers typically offer grilles as optional add-ons. While they serve no functional purpose today, grilles provide aesthetic appeal and can enhance a home’s architectural character. External grilles require removal for window cleaning, making between-the-glass options more practical for busy homeowners.
Why the Terminology Confusion?
Few window companies distinguish between muntins, mullions, and stiles anymore because most buyers don’t differentiate these terms. This historical distinction rarely matters in practical window shopping.
Major manufacturers like Pella use the universal term “grille” for all these elements—whether muntins, grids, windowpane dividers, or mullions. This simplified terminology eliminates confusion while accurately describing components that now serve primarily decorative purposes.
The terms “stiles” and “rails” refer to the outer vertical and horizontal frame pieces, completing the window terminology puzzle.
Common Misspellings
“Muntin” sometimes loses its “n,” becoming “mutin” or “mutton”—both incorrect. “Mutin” isn’t a word, and “mutton” refers exclusively to sheep meat, having nothing to do with windows or buildings.
Choosing the Right Windows for Your Home
Whether you prefer clean, modern lines or traditional divided-lite aesthetics, understanding these architectural elements helps you make informed decisions. Aluclad window features and benefits combine enhanced durability with thermal performance, making them popular choices for homeowners seeking both beauty and functionality.
Ready to upgrade your windows? Expert Windows offers comprehensive window solutions tailored to Irish homes. From energy-efficient designs to custom aesthetic options, we’ll help you find the perfect windows that match your style and performance needs. Contact Expert Windows today for a free consultation and discover how quality windows can transform your home’s comfort, appearance, and value.
