> Only a limited set of frequencies is used in music. All these frequencies (that exist in music) can be played by a piano.
In addition to microtonality, many instruments (and the human voice) are capable of infinite tuning, and musicians will deviate from the "correct" note (as played by an even-tempered instrument) based on chord structure, voice leading, etc.
> To describe tones that exist in music, we can describe tones that exist on a piano. There are only two rules:
> the first key must play a tone of 27.5 Hz each next key must play a tone 6% higher than the previous key
(This is not to mention that at a 1.06x increase per half step you're going to be sharp by 1 cent / octave which starts to add up).