Sold under many names
Nailing fin
A thin flange around a new-construction window that nails flat to the wall sheathing so the unit can be flashed into the wall.
What it means for you
It tells you which install you're getting. A finned unit means the siding comes off for a full, flashed new-construction install; a finless (block-frame) unit drops into the existing opening. That choice drives both water-tightness and cost.
Where you'll see it
In “new-construction vs replacement” and full-frame discussions.
Also sold as: nail fin · mounting flange · nailing flange
Related terms
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