This is the only true way to check for straightness before leveling your frets. Essential for proper truss rod adjustment.3mmx38mmx597mm Machined Edge 6061 Aluminum. One side is for 86.36 cm (Fender) scale basses, and the other side is for 88.9cm (most 5 string) scale basses. CNC precision machined. Good to use with 18 or 20 fret necks..doesn't matter. Use with strings on or off. Notched to the 19th 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.