The most common guitar is 4mm. By straightening the neck of the guitar, you can adjust the radius of the fingerboard.
Graphtech TUSQ 73.8mm x 8.4mm Martin acoustic guitar low action saddle BQ-9161
Vintage 80s Korea Black Micro Tilt Adjustment Guitar Neck Plate & Screws #1,
Potentiometer Cap Volume Adjustment Guitar Knob For Electric Bass For Electric
Truss Rod Adjustment Wrench Set for Acoustic Guitars Includes 4mm & 5mm Tools
