Why do guitars have different position markers than almost all other fretted instruments?

Question

If you look at banjos, mandolins, ukuleles, and balalaikas, almost all of them have a position marker at the 10th fret whereas almost all guitars mark the 9th fret instead. What accounts for the difference?

Answer 1

If you figure there is a reason, I would expect it to be a harmonic reason. And of the various harmonic reasons you could be concerned with, I would expect things like tonal degrees, major triad tones, and tonic/dominant relationships are the important concerns.

The 12th fret marker is obvious, it provides the octave point for each string.

The 5th and 7th frets also seem obvious, they mark a P4 and P5 on each string, tonal degrees relative to the string.

When strings are tuned by P5 then the higher string of a pair fretted at the 10th fret will form a minor seventh above the higher string, which could be understood as part of a dominant seventh chord. Ex. string pair G D, D string at the 10th fret is C, the seventh of G:V7.

When strings are tunes by P4 then the lower string of a pair fretted at the 9th fret forms a M6 above the lower string, but is also a M3 above the higher string. Taken together you have the tones of a major triad rooted on the higher string. Ex. strings E A, A string at the 9th fret is C#, E C# A are the tones of an A major triad rooted on the A string.

I don't know if there is a definitive answer to this question. I would expect historic examples could be found of various instruments with markers at positions differing from what is common today. But if you want some sensible way to understand their positions, you can look at the harmonic relationship to adjacent strings.

FWIW, the 3rd fret on guitar, on the low and high E strings, marks G, which makes sense, because standard tuning is roughly designed around G major.