Essential for proper truss rod adjustment. 1/8″x1.5″x17″ with machined edges  6061 Aluminum. For short scale Fenders only, NOT for Tele or Strat. One side is for 24″ (Fender Jaguar) scale guitars, and the other side is for 22 1/2” (Fender Mustang / Duosonic) scale guitars. CNC precision machined for a flatness of +/- .001″ 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.