The perpendicular fret distance is the ratio of distances along the first and last string that fall on a line perpendicular to the midline of the neck. This is used to control the angle of the nut, frets and bridge.

Traditionally this property of non-parallel-ly fretted fretboards is measured by assigning a "perpendicular fret". "Perpendicular distance" avoids two problems with the "perpendicular fret" method. First, it is possible that no fret falls into this perpendicular position. With "perpendicular distance" we avoid fractional frets. Second, it is possible and even likely with non-equal temperament fretboards that as a fret crosses the fretboard it will fall at different ratios along the strings. With "perpendicular distance" we avoid complex calculations and have more predictable results.

A value of 0 results in a perpendicular nut. A value of 1 results in a perpendicular bridge. The default 0.5 results in a perpendicular octave fret. To calculate an appropriate value for any fret, simply divide the distance of the fret from the nut by the total length of the string. In twelve tone equal temperament the values look like this:

Fret P.D. Fret P.D. 1 0.05613 13 0.52806 2 0.10910 14 0.55455 3 0.15910 15 0.57955 4 0.20630 16 0.60315 5 0.25085 17 0.62542 6 0.29289 18 0.64645 7 0.33258 19 0.66629 8 0.37004 20 0.68502 9 0.40540 21 0.70270 10 0.43877 22 0.71938 11 0.47027 23 0.73513 12 0.50000 24 0.75000