They're specially sized for raising the string height at the bass and treble sides independently, with clearance for the mounting screws and for "bullet" truss rod adjustment. No woodworking! Made of bronze, so they won't dampen the instrument's tone or sustain.
Set of 8 includes two each of .004", .006", .016" and .024" thicknesses (.10mm, .15mm, .41mm and .61mm). A must for more accurate guitar setups!