The Fret Rocker is used to check for high or low frets after a fret job or to diagnose buzzing, the 4 different length sides allow it to span 3 frets anywhere on the fretboard, if it rocks on any of the 3 frets it indicates a problem area.

The side lengths measure: 100 mm (3.9"), 74.5 mm (2.9"), 49.5 mm (1.9") and 33.3 mm (1.3").