This unique tool allows exact thickness measurements of stringed instruments to be made without disassembly. The small counter-magnet is passed through the instrument's f-hole and follows the gauge around every edge and curve, allowing the exact thickness to be measured at any place on the body. Accurate to within 0.1 mm, calibrated for making measurements while holding the gauge in a vertical position (horizontal use will result in a measurement error of +3%). Quick and easy to use, with no risk of damaging even the most sensitive varnishes. Ideal for the study of old instruments, locating old crack repairs, making sound adjustments, etc.