Radius gauge

A set of curved templates, each ground to a stated fingerboard radius, used to check that a board's curvature matches its nominal figure.

The gauge is laid across the frets, and the template that sits flush along the whole width identifies the radius; a mismatch shows up as rocking or as a gap under one edge or the other. It matters because fretwire is pre-bent to a radius, and saddles and nut slots follow the board's curve, so a board that is not what it claims makes setup targets meaningless. The same idea is used to set the string height across the fretboard when a saddle is filed.

Example: setting the individual saddles of a bridge to a radius gauge that matches the neck keeps the strings a consistent height across the board.

Radius gauge

See also