The difference in pitch between any two musical notes. The interval corresponds to a frequency ratio, but intervals are referred to by name and are based on the octave in a diatonic scale. For example, aA fifth is the interval of the diatonic scale between the tonic and the fifth note of the scale, etc.; the fifth has a frequency ratio is 1.5:1, or 3:2. The next most consonant interval to the octave, the second harmonic of the higher tone will have the same frequency as the third harmonic of the lower tone so that the two tones blend together well. The musical interval between the second and third harmonics of a musical sound is a fifth. For the key of C:

The musical interval between the third and fourth harmonics is a perfect fourth, in the diatonic scale, with a frequency ratio of 3:4, and the interval between the fourth and fifth harmonics is a major third which has a frequency ratio of 5:4. The next two harmonics, the fifth and sixth, span a musical interval of a minor third. For the key in the mud