Yibuy Yibuy Guitar Fret Rocker Leveling Luthier Tool for Guitar Violin Mandolin The Fret Rocker can pinpoint those pesky high frets, and check the final height after you redressed or replaced them. Make it versatile for any fretted instrument, use for guitar, bass, mandolin, banjo,violin.. The length of the sides makes it easy to check all of the frets right the way up the fingerboard. It lets you do more accurate fret work for better playability.