View the full lesson at Tune Your Guitar Using Harmonics | JustinGuitar
I’m getting “This video is unavailable”, although I was able to find it on JG’s YT
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Hey everyone - thanks for letting us know. I just fixed the issue; the link was so wrong! 
All good now. Sorry for the hassle! Cheers
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I haven’t gotten into the theoretical side of harmonics yet, but I’m curious to know the answer to these questions.
- Why does the harmonic on S6/F5 sound the same as the one on S5/F7?
- Why can’t we use the same method for the B-string, using S3/F5 and S2/F8? If we shift everything up one fret on the B-string, the underlying notes are still a 5th apart.
I’m assuming harmonics and notes are a different thing then?
It’s science, yo.
Basically, the 7th fret harmonic raises the frequency of vibration of the string by a factor of 3. This is because the 7th fret is located 1/3 of the way along the full length of the string. Similarly, the 5th fret harmonic raises the frequency by a factor of 4.
The frequency of the 6th string is 82 Hz (I looked it up), so the S6/F5 harmonic rings out at 82 x 4 = 328 Hz.
The frequency of the open 5th string is 110 Hz, which multiplied by 3 (i.e. 7th fret harmonic) is 330 Hz, which is effectively equal to the 328 Hz we got before. Hence, same pitch.
The technique works for all strings that are tuned “a fourth” apart because the ratio of the frequencies of those strings is 4/3 and the ratio of the harmonic “factors” for 5th and 7th frets is also 4/3.
And, yes, harmonics and notes are a different thing.
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That’s very helpful, much appreciated. I’m going to use this knowledge to answer my second question myself. If I can find the base frequency of the B-string, than all I need is a bit of math to work out the harmonic positions.
I’ll let you know how it works out. Might take me a while though. 
Sure. I’ll give you a hint: B string frequency is 247 Hz.
Harmonics only ring out where the sound wave crosses the mid point of the cycle. So you would need a string that cycles the same as John explained in his post to match the B string. Unfortunately because the B string is tuned to a 3rd you won’t get a natural Harmonic on the guitar to match.
You could do it with artificial harmonics
Ok, I’ve covered the basics of natural harmonics now and I think I got it.
Natural harmonics can be found at a number of frets that divide the string in equal parts.
The harmonics at these frets (5, 7, 9, 12 being the easiest ones) sound like the octave, the 5th, the 3rd and the double octave of the open string respectively.
That’s why S6/F5 (an octave of the open E) sounds the same as S5/F7 (the 5th of the open A).
It also explains why we can’t use this method for the B-string. S3/F5 sounds like the octave of the open G, which doesn’t match any of the harmonics found on the B-string.
That’s why we have to use S6/F7 (the 5th of the open E) and the open B-string. But we could also use S3/F4 or S3/F9, both a 3rd of the open G-string, and the open B-string.
Easy right! 