Designed to check guitar neck straightness of the fretboard and not The frets. This is the only true way to check for straightness before leveling your frets. Essential for proper truss rod adjustment. .125"x1.5"x18.5" Machined Edge Aluminum. One side is for 25.4" long scale Martin Guitars, and the other side is for 24.9" short scale Martin Guitars. Keep your guitars fret buzz free. I wanted to make this tool affordable not only for guitar techs but players as well, so there is no fancy engraving, etc. 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. Place on top of the frets (off of the cut notches) to use as a standard Straightedge to check fret wear and level.