The fretboard radius is the curvature of the fretboard. Electric guitars generally have a rounder radius (9.5''), while acoustic instruments have a flatter radius (16''). The neck radius is a significant factor in why certain capos sound good on some guitars but not others. Suppose the capo doesn't fit the radius of a guitar's neck. In that case, it could be applying uneven pressure across the strings causing intonation issues or even dead notes.