You read a strange subset of musical tuning literature and picked up a weird misunderstanding. Harmonics means all the multiples of the fundamental. See https:

You read a strange subset of musical tuning literature and picked up a weird misunderstanding. Harmonics means all the multiples of the fundamental. See https://en.wikipedia.org/wiki/Harmonic_series_(music) https://en.wikipedia.org/wiki/Harmonic_series_(music) People talk about primes because of their significance in mapping harmonic relationships. Please don't talk about "overtonal stacking" that's not a thing. "Overtones" means any of the actual tones in a sound that are higher than the lowest one, whether the sound is harmonic or not. Your piano claim is wrong also. What's happening is that the 4th harmonic of the major ninth matches the 9th harmonic of the lower note. And you do not fill in the "missing 5th" unless you happen to have trained yourself to imagine it.