Any two musical notes with fundamental frequencies in a ratio equal to 2n (n is any integer) are perceived as very similar and represent the simplest interval in music – an octave. Human pitch perception is periodic so that “color” or chroma of all the notes that are an octave apart seem circularly equivalent and brings them together into one pitch class.

12-Tone Equal Temperament

(3/2)12 ≈ 129.74634 ~ 128 = 27
Harmonic series