Απο λιγο ψαξιμο βρηκα αυτες τις 2 απαντησεις...
3rd fret notes: (G,C,F,Bb,D,G) (only 1 flat, all diatonic to F and Bb), and 2 open string notes are available (D,G)
5th fret notes: (A,D,G,C,E,A) (no sharps/flats, diatonic to C, and G, and F), and 4 open string notes are available
7th fret notes: (B,E,A,D,F#,B) (1 sharp, all diatonic to G, D, and A), and 4 open string notes are available
9th fret notes: (C#,F#,B,E,G#,C#) (3 sharps, all diatonic to E, and B), and 2 open string notes available.
12 fret notes...important for obvious reasons.
και
The explanation (IMO) is much as others have said:
Frets 12, 7 and 5 mark the natural divisions of the string into perfect intervals (octave = 1/2, P5 = 2/3, P4 = 3/4). 0-5 and 7-12 are the classic "tetrachords" dating back to Greek modal theory.
Frets 3 and 9 then make symmetrical markers between 0-12 (either side of the fret 6 mid-point). (9 actually indicates another string division, 2/5; and 3rd fret is roughly 1/6 - but IMO this is not significant.)
(While I disagree with skinnytommy about the guitar being originally drop D, that isn't necessary to support the idea of the dots representing dorian mode, or a dorian pentatonic: 0-3-5-7-9-12 = m3, P4, P5, M6, 8. Dorian was mode I in the middle ages. Of course, that was well before the guitar existed! IMO, the matching symmetry of both dorian mode and the guitar fretboard markers is a coincidence. Is symmetry is an ideal, then it would lead to both choices independently.)
However, I have seen some instruments with a marker at 10 instead of 9, and others with a marker at 1. Obviously the latter is not to aid navigation ("duh, where's fret 1 then... phew thank god for that marker!..."
) - it's merely decorative.
Of course the purpose of the markers in general is to aid navigation, but those specific frets are chosen for the above reasons: a mix of musical (string division, perfect intervals) and decorative (symmetry).