Two sizes for one opening
A mulled unit is two or more windows joined by a mullion — the mull post — into one assembly: a twin, a triple, a picture window with an operable on each side, a stack with a transom. The customer orders an overall size, and that overall size is divided into openings.
- The sold size of an opening is measured centerline to centerline: to the middle of each mull post beside it. The sold sizes of the openings add up exactly to the overall size.
- The frame size is the window actually built: the sold size less half a mull post on every side that is shared with a neighbour. The outside edges of the assembly lose nothing.
A worked example
Take a 108" × 60" triple with three equal openings and, to keep the numbers easy, a 1" mull post.
| Opening | Sold width | Shared sides | Frame width | Height |
|---|---|---|---|---|
| Left | 36" | right only | 35 1/2" | 60" |
| Centre | 36" | left and right | 35" | 60" |
| Right | 36" | left only | 35 1/2" | 60" |
| Total | 108" | 106" + two 1" posts = 108" |
The check at the bottom is the one to make every time: frames plus posts equal the overall size. If they don't, one of the openings has been built at its sold size.
The same rule applies vertically. In a stacked unit — a picture window under a transom, say — the heights lose half a post where they meet, and the widths are untouched.
Unequal splits
Openings are not always equal. Split the same 108" unit 2:1 with a 1 1/4" post and the sold widths are 72" and 36"; the frames are 71 3/8" and 35 3/8", and 71 3/8 + 35 3/8 + 1 1/4 = 108 again. When a split produces a fraction finer than your plant cuts, round the frame down to your cutting increment (usually a sixteenth) so the assembly never comes out over size.
Which size reads which?
This is where most systems go wrong. The rule is simple once it is written down:
| Purpose | Reads | Why |
|---|---|---|
| Price from a size book | Sold size | The price grid was built for openings measured the way they are sold. |
| United inches (width + height) | Sold size | Per-UI charges, such as a color upcharge, bill the opening. |
| Frame and sash cut lengths | Frame size | Cut from the sold size, every shared side is half a post long. |
| Glass sizes | Frame size | The glass sits in the frame that is built. |
| Glass area rules (tempering, area charges) | Frame size | A lite's real area decides whether it is over a size limit. |
| Labels and paperwork | Frame size | The bench needs the size of the thing in its hands. |
The tempering line is the subtle one. A lite can be over a 30 sq ft annealed limit measured in its centerline cell and under it in the frame that is actually built. Judge it on the frame, and tempered glass is charged and ordered only where it is really needed. (More in our guide to tempered glass rules.)
Try it: our free mulled window size calculator works out the centerline and frame size of every opening for any overall size, number of openings and mull post.
The three common mistakes
- Building at the sold size. Each frame comes out long by half a post on every shared side, and the assembly will not go together at the overall size.
- Pricing at the frame size. The size book reads a smaller opening than was sold, and every mull is undercharged by a few dollars — invisibly, forever.
- Storing one size and deriving the other by hand. When a rep resizes the assembly, the hand-calculated size is the one that gets forgotten.
How Extrusia handles it
In Extrusia each series carries its mull post width. When an assembly is configured, every opening gets both sizes — sold to the nearest 1/32" on each mulled side, frame floored to 1/16" — and the price engine reads the sold size for size books and united inches while every cut list, glass order, area rule and label reads the frame. A series with a post width of zero treats every opening as its own frame, so storefront-style assemblies are unaffected. Nobody has to remember which size to use; the system can only use the right one.