Sold under many names
Balance (window)
The hidden spring mechanism inside a hung window's jambs that counterweights the sash so it stays put wherever you stop it.
What it means for you
When a double-hung sash “won't stay up” and slams shut, a broken balance — not the whole window — is almost always the cause, and it's a cheap, common repair.
Where you'll see it
Hidden in the side jambs; the usual fix behind a “sash won't stay open” complaint.
Also sold as: spiral balance · block-and-tackle balance
Related terms
Reading this on your own quote?
Run the 30-second check — where your number lands against real NC ranges, plus the red flags.
Check my quote →