What does fretboard radius mean, and how does a 12-inch radius differ from a 16-inch one?—Hank Tortora
The term fretboard radius describes the curvature of the playing surface of the neck. Nearly all steel-string guitars have fretboards that have a crowned center—thickest in the middle, and slightly thinner along the edges. This is meant to make the neck easier to play and more friendly to the fingers, though it’s worth noting that classical guitars nearly always have flat fretboards, which seems to present no hindrance to highly technical playing.
The numbers used to describe radius are a measurement of the theoretical circle defined by the fretboard curve. If you were to trace the curvature of the fretboard, and project that same curvature beyond the neck itself, it would form a circle whose radius matched the specified number. Or, to put it another way, if you had a perfect circle of fretwire with a diameter of 24 inches, then a piece cut from that wire would perfectly fit the curvature of a 12-inch fretboard radius.
A smaller radius will have a more pronounced curvature. This is most often encountered on vintage or vintage-spec Fender-style guitars with their 7.25-inch radius. This is substantially rounder than most modern guitars and can present issues with playability, particularly for players who like to bend strings. You can visualize these issues easily by taking a can, bottle, or any other cylindrical object and a straightedge or ruler. Lay the ruler along the can, parallel to its long axis, and you’ll see that it touches along the entire length. This means that the can’s surface would be perfectly level relative to the ruler, just as we hope our frets will be relative to the strings.
But the problem arises because this perfect surface only exists when the ruler is parallel to the can—as soon as you pivot the ruler at all, it begins to rock around on a single point on the can’s surface. Because the nut is narrower than the bridge, the strings on a guitar are not all parallel to the neck centerline. Therefore, the fretboard surface intersected by the string is not level. In most cases, this issue is small enough to be theoretical, and the guitar will still play cleanly without buzzing. However, on guitars with very small radii, this can manifest as the notes choking or fretting out, particularly when you bend strings.
Things get a bit more complex with some modern guitars, which use a so-called compound radius. These fretboards generally have a smaller radius with more curvature at the nut end, gradually flattening to a larger radius at the higher frets. This approach is meant to idealize comfort for different kinds of playing in different areas of the neck—chord playing in the lower positions, and lead playing higher up. It also addresses the geometry issue described above, by effectively making a fretboard surface that is conical rather than cylindrical.
It is easy to repeat the experiment above with a conical object, and illustrate that it’s possible to find a perfect level surface with rulers in multiple positions on a cone, even when they are not parallel. These fretboards are more time-consuming to produce, and their geometrical advantages are generally only meaningful for players who seek out an extremely low setup. For typical low-to-medium acoustic guitar action, a cylindrical fretboard can certainly be set up to play cleanly and without buzzes.
The reality is that most radii are very close in curvature. Let’s consider two of the most common radii, 12- and 16-inch. At the nut position of a typical acoustic guitar (using a 1.75-inch nut width), the 12-inch radius fretboard has a difference in thickness of .032 from center to edge, whereas the 16-inch shows a difference of .024—a net difference of .008. That’s roughly the thickness of the lacquer on the back of the neck, and thinner than a light-gauge electric guitar string. This is a very small difference, and one that could be easily overlooked by even a serious long-term player.
Except in the extreme cases of the vintage 7.25-inch electrics, or the dead-flat classicals, the differences between most common radii are usually fairly subtle, and I would hazard a guess that four out of five of my clients don’t even know which radius their fretboard has. If you find a guitar that just feels especially good, it’s always a good idea to look up the specs and take note—but plenty of folks go their whole lives never needing to know what that particular number means.
