JBG, that's true, of course. But I wonder who was the first to recognize this. I would nominate (again) Pythagoras. He did discover scales, including (that early) the 12-tone scale. I imagine he at least experimented with how certain notes sounded when played together. But he thought certain numbers had moral value or supernatural characteristics. I think he was the one that started the idea the the number 13 is unlucky, so he probably didn't play many thirteenth chords.

"Okay, now if I take the square root of the hypotenuse. . . ."

Don Firth