One side is for 30″ Scale Basses, and the other side is for 30.5″ Scale Basses. CNC precision machined for a flatness and accuracy +/- .001″ Good to use with 18 or 22 fret necks, doesn’t matter. Use with strings on or off. Notched to cover the entire fret board, 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.
Measurement from the nut to the 12th fret should be 15″ or 15.25″ +/- 1/16″ to use this tool. Feel free to ask which tool is right for your instrument.