Sold under many names
Shims
Thin tapered wedges the installer slips behind the frame to set the window plumb, level, and square before fastening it.
What it means for you
Small pieces, big consequences. Windows shimmed in pairs and checked for square operate smoothly and seal evenly; a racked, badly shimmed unit binds, won't lock, and wears early. Over-tightening screws where shims are missing can even bow the frame.
Where you'll see it
Hidden behind the trim; you feel the result when a sash glides or sticks.
Also sold as: shimming
Related terms
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