Architecture Vocabulary: Muntins vs. Mullions

Thanks to the Internet and numerous shows focused on home improvement and real estate, even a casual design enthusiast can build up a pretty robust vocabulary of architectural terms. But beyond terms like “beadboard” and “crown molding,” there are some words and phrases that can trip up even the most experienced of architects. Today, we’re taking this window of opportunity to dig into two of the more often confused architecture words (even among industry pros): Mullion and Muntin

A mullion is a vertical element between two window frames. This provides structure between window frames, accomplishing a larger mass of windows as one. In the above photo, the mullions bring together five window frames in a semi-circle to maximize the room’s view.

A muntin is a rabbeted (or recessed) member for holding edges of window panes within a window sash. Muntins are also sometimes called glazing bars or sash bars. Patterns in today’s muntins are typically decorative and can range from very simple to complex.

Historic origins & current applications

Historically, panes of glass were only available in small sizes and very expensive. That made muntins crucial, functional elements of window design, and explains why historic homes typically feature windows with multiple panes of glass. In the 1800s, the cylinder process and plate glass technique allowed for larger panes to be produced in a much more cost-effective manner, and window design began to evolve accordingly.

Today, multi-pane windows with traditional muntins are called True Divided Light (or TDL) windows. Aesthetically, TDL windows offer an authentic, historic look, which can be important when constructing or renovating in a historic district. Functionally, TDL windows can help strengthen and stabilize the window sash and it can be more cost-effective to repair or replace since these windows use smaller panes of glass.

For those who desire a more energy-efficient solution but still want a historic look, Simulated Divided Light (or SDL) windows are another attractive option. SDL windows include a grille that is made to look like a muntin but is located on the inside of the window. This allows for easier cleaning and greater efficiency while maintaining the classic look of a multi-pane window with muntins.

At TMS Architects and Interiors, we pride ourselves on our ability to blend historic style with top of the line, sustainable technology in our home designs. To get started on your next home, renovation, or interior design project, contact us today.