The Elmer Guitar Double Sided Notched Straight Edge sits over the frets on to the fretboard wood, allowing you to check the relief of the neck for truss rod adjustments or check for fretboard humps at the heel etc. without worn or raised frets distorting your measurements. The straight edge features two sets of notches, one set is for 25.5" scale length guitars (e.g. Fender Stratocaster, Telecaster etc.) and will also fit a 650 mm classical scale length, the other set is for 24.75" to 25" scale length guitars (e.g. Gibson and PRS). The straight edge is placed starting on the 2nd fret and can also be placed up on to the frets to quickly check if they are level.