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→Harmonics
To demonstrate, the 6th string should be tuned to E, or 82.5 HZ. Sounding the 4th harmonic would give us 4x that pitch. 4 * 82.5 = 330. The 5th string should be tuned to A or 110 HZ. The 3rd harmonic is 3x that pitch. 3 x 110 = 330. So 6th string 4th harmonic = 5th string 3rd harmonic = 330 HZ. 82.5 (E) x 4/3 (4th interval) = 110 (A).
So if we sound the ''5th fret harmonic of the 6th string'' and the ''7th fret harmonic of the 5th string'', the same note should be ringing out. This frees up the left hand to rotate the tuners to achieve the same note. And like open strings, harmonics tend to sustain longer. This works on every pair of strings except the 2nd and 3rd strings. For these we can use the 5th harmonic of the 3rd string and the 3rd harmonic of the 1st string. That's the 3.9 or 8.8 fret harmonic on 3rd string and 7th fret harmonic on the 1st string.
3rd string G is 196 HZ. 196 x 5 = 980. 1st string E is 330 HZ. 330 x 3 = 990 HZ. But wait - 980 != 990. Unfortunate, but close enough for our purposes. If we tune the 1st string 3rd harmonic to 980, that means the 1st string is now 326 and 2/3 HZ. That's only 3 1/3 HZ away from our target of 330 HZ, and the human ear has trouble differentiating pitches within such a small window (approximately 1% off).
e----------------<7>----<7>-
b-----------------------<5>-
g-----------<7>--<38.8>------
d------<7>--<5>-------------
a-<7>--<5>------------------