Wood shims are thin, triangular blocks of wood used as spacers to make minor adjustments to the position of two adjacent objects. In window and door installation, they’re often used to make sure the jamb is square inside the rough opening of the house frame before fastening.
The best method for using wood shims is to insert them in pairs between the two surfaces you’re trying to move or support. When shimming a door, for example, shims are inserted from each side of the door jamb so that their thin ends meet and overlap in the middle. By sliding the shims further into the gap, the wood pieces slide over each other and push the shimming surface outward, creating the thickness of two shims. This dual-shim method also provides a flat surface to work with, rather than using a single triangular shim from one side, which creates a “teeter-totter” effect against the surface being shimmed. –– M Weber