Muntin vs. Mullion! What’s the Difference?

French door with lockbox and muntins as part of a window wall with steel mullions

I would venture to say that most people don’t even know the words muntin and mullion, much less know what the difference is.  In fact, as I am typing this, the automatic spelling checker in Microsoft Word has identified “muntins” as a misspelling.  Wow, even a mega-corporation like Microsoft doesn’t know what a muntin is.  Yup, even the singular form used in the last sentence popped up as an error.

Both words are used in the window and door industry and describe dividing and joining glass and frames.  This idea of both dividing and joining has always struck me as peculiar, which is it?  Well, it is a matter of perspective.  Let’s take mullions.  If you take the point of view of the window or door opening, the mullion divides it into smaller blocks so that the individual window or door unit isn’t so large.  If the opening were twenty feet wide, it wouldn’t be so easy to build a single unit that big, transport it to the jobsite without damage, and successfully install it.  The smarter thing to do, even if it is fixed, is to make at least two or three fixed frame windows and connect them together by adding a mullion at the intersections.  So here we see the window frames perspective.  The mullion is the splice piece that allows two or more window/door frames to be joined to one another.  Dividing the opening and joining frames together are the same thing using mullions.  The engineering company that I work for is named Mullionz LLC and the tag line is “it’s a matter of perspective”.

Another useful thing about mullions is that they can join dissimilar items together.  Fixed and operable windows can be joined to doors of any variety within the same opening by incorporating mullions.  The use of the word mullions dates to the Middle Ages, and they were exclusively vertical members.  As reinforcement, they absorb windload from the glass/frame surface where the opening is intrinsically weak and transfer it to the head and sill of the structure.  While window and door people usually use the term mullion to describe both vertical and horizontal members, the term transom is the most correct name of horizontal “mullions”.

Muntins on the other hand divide, reinforce and join glass within a single window or sash frame.  These are the small vertical and horizontal bars that change large pieces of glass into small “divided lites”.  Their origin is also very old and made necessary because glass could not be made in very large sizes without breaking.  To fill a larger frame with glass, small pieces needed to be joined together with muntins.  Since the glass joints were also weak relative to a large frame opening, they sometimes needed to also add depth to resist windload.

Essentially, muntins are a specialized mullion.  I often work with architects who say that they need mullions in the window when in fact they are talking about muntins.  I often correct them because I am a wiseguy.  It isn’t a cardinal sin, but irritating accuracy is an engineer’s plight.  So here is the deal; mullions divide openings while muntins divide glass.  It’s that simple.

This is a funny topic for the inventor of 2Fold® since we don’t even offer muntins in our system now, nor do we use mullions since bi-folding doors only have single frames within huge openings.  As we introduce the single and French door versions of the system soon, they will also be introduced.  Since 2Fold™ celebrates large expanses of uninterrupted glass, muntins seem antithetical, however, I’ll eventually cave into demand for them at some point, I’m sure.  In fact, I have them designed already.

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