Designed to check Bass/Guitar guitar neck straightness of the fretboard and not Just the frets. This is the only true way to check for straightness before leveling your frets. Essential for proper truss rod adjustment. 1/8”x1.5"x21" Machined Edge 6061 Aluminum. This is very rigid and will last for years. One side is for 30" Short Scale Basses, and the other side is for 30.5" scale basses. CNC precision machined for a flatness of +/- .001" Good to use with 18 or 24 fret necks....doesn't matter. Use with strings on or off. Notched to cover the entire fretboard, with clearance for the nut on the other end. . 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.