“Bar frets were used on CF Martin guitars made before 1934. Part of the function of this type of fret is to act as a wedge and stiffen the neck. The change to the modern “T” cross-section frets was accompanied by the addition of steel neck reinforcement. The first photo to the left illustrates (from side-view) the difference between those profiles. The other photos are of a 1929 OO-28 Martin fingerboard. Bar frets are difficult to work with and often require ‘shims’ to get the needed profile above the fingerboard.”

— Marty Lanham