Window Mullions Explained: What They Are & When to Use Them

Window mullions and muntins create architectural style through vertical and horizontal dividers—but most people confuse these terms and miss important differences that affect function and cost.

Understanding Window Dividers

Walk into any hardware store and ask about window mullions, and you’ll likely get three different answers. Even contractors mix up mullions, muntins, and grilles. This confusion costs homeowners money when ordering replacement windows or matching historic homes.

The terminology matters because each type of window divider serves different purposes. Some provide structural support for large bay windows or French doors. Others create smaller panes purely for looks. Bay Area homeowners need clarity—Victorian windows in San Francisco and Craftsman-style bungalows in Berkeley demand specific approaches.

What Are Mullions on a Window?

Window mullions are vertical or horizontal bars that divide a window frame into separate sections. Each section holds its own window panes or glass units. Think of the center post between two casement windows or the bars creating sections in bay windows.

Mullions provide structural support for large window openings. Before modern glass manufacturing could produce big panes, mullions divided the weight across multiple smaller sections. Today, they still serve structural roles while defining architectural style.

You’ll find mullions in French doors separating individual door panels, in storefront windows creating display sections, and in traditional windows where multiple units sit within one frame opening.

What Are the Dividers on Windows Called?

The answer depends on what they’re dividing:

Mullions divide the window frame itself into sections. These are structural elements that create separate window units within one opening. Vertical mullions run up and down. Horizontal mullions run side to side.

Muntins divide individual panes of glass within a single window. These create the grid pattern you see in colonial windows. Muntins can be structural (true divided light) or purely decorative grids.

Grilles are decorative overlays that simulate divided panes without actually separating the glass. Grilles between glass sit inside sealed units for easy cleaning.

What is the Difference Between Muntin and Mullion?

Mullions separate window frames. Muntins separate window glass. That’s the key distinction most people miss.

A mullion creates multiple window units. Picture a large opening with three casement windows side by side—the vertical bars between each window are mullions. They’re part of the window frame system and provide structural support.

Muntins create patterns within a single window. A six-over-six colonial window has muntins dividing the glass into smaller panes. These window grids can be authentic wood pieces holding individual glass sections (true divided light) or decorative grids attached to larger panes (simulated divided light).

The confusion arises because both create divided appearances and appear in traditional windows. Regional terminology makes it worse—some areas call everything “mullions” while others say “muntins” for all dividers.

What is the Use of a Mullion?

Window mullions serve three main purposes in Bay Area homes:

Structural support: Mullions carry the weight of large window installations. Bay windows need mullions between sections. Multi-pane windows use mullions to distribute loads across the window frame. Before modern manufacturing, mullions enabled large openings.

Functional flexibility: Mullions let you combine different window types in one opening. One section opens for ventilation, while the other stays fixed. This creates separate operable sections for better airflow control.

Architectural character: Victorian windows in Alameda feature vertical mullions that define the style. Craftsman homes in Oakland use horizontal mullions for emphasis. Mediterranean designs incorporate arched mullions. Mullions break up expansive glass and create visual interest that matches your home’s character.

Types of Window Grids and Dividers

True divided light uses individual glass panes separated by wood or vinyl muntins. This authentic approach suits historic homes but costs more and reduces energy efficiency. Each small pane needs to be sealed, creating more points for air leakage.

Simulated divided light attaches decorative grids to the glass exterior with a spacer bar between double panes. This looks authentic on the outside while improving energy efficiency. It’s popular for Victorian restorations in the Bay Area.

Grilles between glass seal window grids inside the double-pane unit. You get the divided look without cleaning hassles. This delivers the best energy performance and works well for vinyl windows and modern installations.

Removable grilles snap onto the interior glass. These offer style flexibility—remove them for cleaning or when you want unobstructed views. They’re the most budget-friendly option for replacement windows.

Window Mullions in Bay Area Homes

Victorian homes throughout San Francisco feature tall windows with vertical mullions and decorative muntins in upper sashes. True divided light creates authentic period character but requires careful maintenance in coastal fog.

Craftsman bungalows in Berkeley and Oakland showcase multi-pane upper sashes with prominent horizontal mullions. The wood muntins add substantial visual weight, defining the style.

Modern Bay Area homes skip mullions entirely or use dramatic structural mullions with large expanses of glass. Clean lines emphasize views rather than divided patterns.

Colonial Revival homes use symmetrical 6-over-6 or 9-over-9 muntin patterns. White-painted traditional windows create formal elegance.

Choosing the Right Window Dividers

Match window dividers to your home’s architectural style first. Victorian and Craftsman homes need authentic-looking divided light patterns. Modern homes work better with minimal or no dividers.

Consider maintenance willingness. True divided light means cleaning many small window panes individually. Grilles between glass require almost no upkeep.

Energy efficiency matters in Bay Area microclimates. Coastal homes dealing with fog need sealed units. Grilles between glass deliver the best performance, while simulated divided light balances authenticity with efficiency.

Budget affects your choice. True divided light costs $1,000 to $2,500 per window installed. Grilles between glass run $450 to $1,150. DIY decorative grids cost under $100, but won’t match the quality of window installation results.

Insight Glass helps Bay Area homeowners select window mullions and muntins that match architectural style while meeting modern performance needs. Our team understands the difference between structural mullions and decorative grids—and which option suits your home and budget.

Talk to our Bay Area window expert about mullion options for your home—request your free estimate today.