the_sauce_baws

Hey abadnit - that actually has to do with the way equal-tempered tuning system "compromises" on certain intervals in order to make every key sound the same! A bit of an obsession of mine :)

Basically the equal-tempered system prioritizes perfect octaves (e.g., all Cs sound exactly in tune with each other) and equality of keys (G Major sounds just as good as C# Major). One of the biggest casualties of this is the size of the major third interval - every equal tempered major third is actually ~14 cents, or ~1/7th of a semitone higher than the perfect major third you'd hear in the overtone series.

So, on a guitar, the B string always sounds out of tune compared to the G string because the two make an equal tempered major third, and is therefore 14 cents wider than perfect! Hope this helps.

Exactly! The "perfect" Major 3rd is the fourth overtone. So over a G you have G -> D -> G again -> B. The B that comes out there is what you're probably instinctively trying to tune the B string to, and also what creates dissonance with the equal tempered B string.