Fretboard/Top Plate Geometry of the Flattop Guitar

by R.M. Mottola¸

Originally published in American Lutherie #111, 2012



The geometric relationship between the fretboard of the flattop guitar and the top of that instrument is an interesting one, if for no other reason than that a number of the critical dimensions of the instrument such as action and bridge height are affected by it. The geometry of these components is also the source of many technical questions from luthiers attempting to make design departures from conventional instruments, and from those who are simply curious about how all this fits together. This article will discuss the topic using basic examples from conventional instruments. The math will be presented for readers who want to make use of it in their design efforts. Diagrams and textual descriptions are also included, and wherever possible I’ve attempted to discuss the practical ramifications of some typical approaches to this aspect of guitar design. This latter part is generally limited to the basic mechanics of how the parts of the instrument fit together. Although it could be fruitful to widen the scope here to include discussion of general mechanics and acoustics of the instrument, I am not dwelling much in these areas. There is enough to cover just discussing the geometry and assembly considerations.

To begin, I’d like to introduce some basic concepts and terminology. Some of these concepts are both generally understood and are described using standard terminology. Terms like bridge height, action, and neck angle are familiar, but let me quickly define each of these as they are used in this article. Bridge height is the nominal height of the bridge from the top of the guitar to the top of the saddle at the instrument’s centerline. Neck angle is the angle between the nominal plane of the top and that of the surface of the neck shaft that contacts the underside of the fingerboard. A positive neck angle means the neck is angled toward the back of the body. Action is the height of the strings above the fret crowns. More specifically (as used here), it is the height of the nominal string-bottom plane over the nominal fret-crown plane.