On fretted instruments, the fret radius can be corrected, i.e. altered to a different desired radius. This assumes that enough fret material is available to make the change.
A few fundamental possibilities are illustrated below.
In the first image, the fret radius and the fingerboard are identical. The fret height is consistent across the entire fingerboard
In the second image, the fret has been filed down in the middle in order to give it a greater radius. This means that the fret radius is larger than the board radius.
In the third image, the fret has been filed down at the edges in order to reduce its radius. This means that the fret radius is now smaller than the board radius.
These techniques are frequently applied by experienced luthiers when working on a fingerboard, e.g. as part of a bending dress or to work with a “ski jump” at the body joint.
The Plek machine gives luthiers a high-precision tool for the detailed control of fret heights under each string at every position along the fingerboard, hence making it possible to precisely implement target radius specifications.
See also Compound Radius for further information.