And you may have also heard of things like 1/3 octave as in 1/3 octave equalizers. One might think that is nothing more than divvying into numerically equal thirds the spaces between two octaves. However it doesn't work like that.
While in a 1/3 octave equalizer, the spacings are three to an octave, they are spaced by the cube root of 2 = 1.2599. An example of frequencies spaced by 1/3 octave would be: 20hz, 25.2, 31.7, 40, 50.4, 63.5, 80 and so forth. Note how every third number (20, 40, 80) is another octave (doubling).