Sold under many names
Mull (mulled unit)
Two or more units — a door with its sidelites, or windows joined side by side — factory-joined into one assembly. “Mulling” is the joining.
What it means for you
A factory-mulled unit is generally stronger and better-sealed than pieces joined on-site, and it installs as one. The trade-off is size: a big mulled assembly can be hard to get through the house and into the opening.
Where you'll see it
Entry-door-with-sidelites orders and multi-window combinations.
Also sold as: factory mull · mulled unit
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