As an example, let’s look at the harmonic series on C, where C is the fundamental note or frequency. The first harmonic plays a C an octave higher, which means that the frequency ratio between octaves is 2:1. The second harmonic is a fifth higher than that, with frequency three times the fundamental frequency. So by dividing by 2, we put the fifth in the same octave as the fundamental, since it is originally an octave too high, and thus to go up a perfect fifth, the ratio is 3/2.