Harmonics 1, 2, 3 and 4 superimposed.

The 1st, 2nd and 4th Harmonics are all represented in Red and correspond to the same note in different octaves. The 3rd Harmonic, shown in blue, is smaller than the fundamental frequency by a factor of 3 and is known as the pure fifth in Just Intonation. If the Fundamental frequency is represented by 1, and the octave is represented by 2, then the pure fifth is represented by 3/2 , or 1.5.

In equal temperament, it is smaller than its just intonation counterpart by 0.1 cents. In other words its value is at 1.49 and it is called the Perfect Fifth.