Essential for proper truss rod adjustment. Machined Edge 6061 Aluminum. One side is for 64.77 (Fender) scale guitars, and the other side is for 62.865 cm (Gibson) scale guitars. CNC precision machined. Good to use with 21, 22 or 24 fret necks..doesn't matter. Use with strings on or off. Notched to the 22nd fret with clearance for more frets and Clearance for the nut on the other end. Covers the entire length for the neck . You can also place on top of the frets (off the cut notches) to use as a Standard Straightedge to check fret wear and level.Neck Check Guitar Tools makes notched straight edges for a wide variety of guitars.