Harmonic@Frets; Starting with A=440 to keep the numbers simple on a standard guitar:

440 = fundamental

880 = 1st harm @ 12th fret, 1/2 length = 2x frequency = 1 octave higher; 1320 = 2nd harm @ 7th fret, 1/3 len = 3x freq ~= 1 oct + 5th higher, (although that should actually 1318.51 which is why that trick of tuning the treble E to the A harmonic doesn't work exactly right);

1760 = 3rd harm @ 5th fret, 1/4 len = 4x freq = 2 octaves higher;

2200 = 4th harmonic @ 4th fret, 1/5 length = 5x frequency, which is sort of close to 2 octaves + 3rd (the nodal point is usually just slightly towards the nut from the 4th fret);

2640 = 5th harm (no fret really, closer to the 3rd fret than the 4th fret), 1/6 len = 6x freq = 2 oct + 5th (or close anyway);

3080 = 6th harm (closer to the 3rd fret than the 2nd fret), and doesn't sound close to anything in 12 step equal tempered tuning, besides that it dies away very quickly;

3520 = 7th harm (closer to the 2nd fret than the 3rd fret), and though its a perfectly good 3 octaves higher, at this point you'd probably get better sound strumming between the nut and the tuning machines.

(If for some reason a guitarist feels compelled to play even higher harmonics, I suggest that (s)he be locked in a room with a million manic mandolinists for a month.)

~S~