My first instrument was recorder, which I taught myself from books. None of the books explained that you can have your fingers in the right place but the music may still be out of tune. When I started going to workshops with professional teachers, they taught us to play a chord, hold it, and listen to it. Basically, you ask yourself, "Does this sound good?" There is an inner sense which tells us whether that chord is right or not, and it was magical the way the group could alter its playing to produce better harmony.

I suspect that many people that play or sing out of tune are not listening to themselves. They are absorbed in the tactile experience and are not using their ears.

There used to be a trio in my town. Husband and wife played banjo and dulcimer beautifully. Fiddler played the fiddle sharp constantly. I'm convinced he was so distracted by the need to finger and bow that he just didn't listen to what was coming out.

(I don't believe that "equal temperament" or "other scales" have anything to do with it. Mudcatters are musical enough to know sour notes when they hear them.)