bar frets vs "t" frets

History of Guitar Bar Frets

“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

1929 OO-28 Martin fingerboard