Leadrise&reg; Set of 9 Understring Radius Gauge for Guitar and Bass Setup Luthier Tools for Bridge Saddle Adjustments 1.Obtain accurate measurements of your instrument's fretboard radius using the bottom radius of the gauge 2.Then use the matching top radius of this gauge to determine the proper height adjustment for the bridge saddles. 3.Radius gauges come in 9 sizes: 7.25" 9.5" 10" 12" 14" 15" 16" 17" 20" 4.These come in the most popular fingerboard radius and make it easy to measure the radius from either above or below the strings. 5.Use these to accurately setup your guitar bridge saddles to match the fingerboard radius.