[palimpsests]

agreed. modern music is "off tune" relative to just intonation - which is barely used largely because we have evolved to appreciate the aesthetics of modulation, something that is extremely limited in just intonation. it's perfectly in tune relative to equal intonation.

[mhauru]

Weellll, yes and no. Yes, modern music is (mostly) in tune with respect to the chosen tuning system. However, there really is a sense in which just intonation is the harmonically most pure tuning system. The reason musical harmony is a thing at all for human ears is that frequencies related by simple ratios have a special sound to them when played together, which we call harmonious or consonant. geofft does an excellent job explaining why this is the case in one of the comments below, and I provide my own complementary explanation in that thread as well. And just intonation preserves this consonance, these simple ratios, exactly. Sure, it comes at the cost of things like modulation that composers and listeners enjoy, I am not contesting that. Nor am I saying that just intonation sounds prettier, that depends on your taste. But for having a chord sound as resonant and consonant as possible, something I think can be fairly described as being "in tune" without reference to a particular tuning system but only to the physics of sound waves, just intonation is the real deal, and everything else is approximating it.

Also, while I agree that things like aesthetics of modulation are one reason why just intonation is such a marginal thing, another big reason is the difficulty of making and tuning instruments for just intonation, when there's in principle an infinity of tones within an interval, and the whole system changes when you change key. I thought this latter reason is an unfortunate one, and something we could try to overcome with digital technology, hence Jintone.