MMdex Guitar Truss Rod Adjustment Long Wrench for Martin Acoustic Guitar To adjust the truss rod on Martin Guitars and most acoustic guitar Two way L-shape truss rod adjustment Neck wrench specialized to the adjusting hole of the guitar neck relatively deep. Adjust the guitar neck straight,suitable for Martin guitar, regardless of the types, the piano body shall be general Size: 12.5*7.5cm/ 5.9*1.77"