Radius refers to the curvature of your guitar's neck. Every neck has a curve to some degree, while some models have more profound curves than others. The neck radius determines how to set up your guitar. For example, the saddle height of each string should be adjusted to compensate for neck radius so that each string is an equal distance from the fretboard. Fortunately, radius gauges enable you to accurately measure the radius of the neck without removing the strings, making for quick, convenient and accurate saddle adjustments.
Examine the radius of your guitar's neck. Radius gauges come in sets with a variety of sizes. It's necessary to fit a few gauges to your guitar before you find the right size. Pick the gauge that is roughly the same radius as your guitar's neck.
Hold the radius gauge between thumb and forefinger. Place it on the fretbard, between strings three and four, parallel to both. Rotate it 90 degrees so that it lies parallel with the fretboard. If there is a gap between the gauge and neck, select a different gauge. Rotate the gauge back 90 degrees to remove it.
Apply gauges until you find the snuggest fit. Once you do, wrap a piece of coloured-tape around the handle so you know which is the correct gauge. The gauge that fits has a bottom radius to the neck's top radius. The gauge's top radius serves as a measure of correct string height.
Slot the gauge between strings three and four so that the bottom edge of the radius touches the top of the bridge. Rotate it so the gauge slides underneath the strings.
Hold the gauge in place. Adjust the height of each saddle so that the string sits snugly on the top edge of the radius gauge, making it the correct height to compensate for the radius of the neck. The method for adjusting the saddle varies according to the make and model of the guitar. For example, with a Telecaster, use a small Phillips screwdriver to tighten the two screws on each edge of the saddle to raise it, or loosen them to lower it. On a guitar with a Floyd Rose tremolo, use a small 3-mm hex key to raise or lower the saddle. Repeat this process for each string, then rotate the gauge 90 degrees and remove it.
Tune your guitar once you have adjusted the saddle since saddle adjustments can slightly influence the tuning.