Shim

Constructed of either wood or plastic, shims may be used on new, used, or vintage electric guitars for a number of reasons. One application is to use a shim to fine-tune the string height at the nut. This is typically done if there is too much string buzz at the lower frets, which indicates that the nut is probably set too deep into the neck. Shims may also be used to make slight adjustments of the neck angle on guitars with bolt-on necks. You won’t know if there’s a shim in your guitar unless you loosen (or unscrew completely) the screws or bolts that hold your neck in place, so it’s best to proceed cautiously and note the positions of any shims you may find.