Guitar Frets are laid out to accomplish an equal tempered division of the octave. Each set of twelve frets represents an octave. The twelfth fret divides the scale length exactly into two halves, and the 24th fret position divides one of those halves in half again. The ratio of the spacing of two consecutive frets is formula_1 (twelfth root of two). In practice, luthiers determine fret positions using the constant 17.817—an approximation to 1/(1-1/formula_1). If the nth fret is a distance x from the bridge, then the distance from the (n+1)th fret to the bridge is x-(x/17.817). Frets are available in
Fret generally perpendicular to the instrument's neck centerline and parallel to each other, on a "fanned" fretboard, the frets are angled (spread like a fan) with only one center fret perpendicular to the neck’s centerline. This gives the lower-pitched strings more length and the higher strings shorter length (comparable to a piano or a harp where heavier strings have different lengths). The idea is to give more accurate tuning and deeper bass. Some think that fanned frets might be more ergonomic. Fanned frets first appeared on the 16th century Orpharion, a variant of the cittern, tuned like a lute. John Starrett