Official Luthiers Forum!

" there is no reason that a classical guitar should have a flat fingerboard. This is an unproductive hold-over from the 18th and 19th century when flat fingerboards were the only kind that luthiers made on plucked string instruments. The primary reason for remaining with the flat fingerboard design is one of ease of manufacture and low cost for the luthier. It does not benefit the player in any way.

Regardless of the width of the neck, some curvature in the fingerboard profile, or radius, is always helpful in making the guitar easier to play. I have read about classical guitars with a very slight curvature, 24-inch or 20-inch (610mm or 510mm). Mine has a 15-inch radius, like a Martin steel-string guitar. A greater curvature, say a 10-inch or 7.5-inch radius like on a Strat, would probably be more than a nylon-string player would want.

The late Thomas Humphrey, builder of the Millenium Guitar, championed a slightly radiused fretboard on his traditionally-proportioned instruments with full-width fingerboards. He mentioned that in addition to comfort and playability, it was helpful to increase the curvature under the bass strings to enable a slightly greater string height to correct problems with string buzzing"