Chapter 2
The perfect fourth is the third most important interval in music.
A perfect fourth above the Tonic is the perfect fourth and a perfect fourth below the Tonic is a perfect fifth.
The Tonic, the Dominant (perfect fifth), and the Sub Dominant (perfect fourth) are the foundation on which harmony in Western music is constructed.
The interval between the third harmonic (3X) and the fourth harmonic (4X) gives the perfect fourth interval.
The frequency ratio of the perfect fourth is 4X/3X=4:3. The interval covers four white positions of the C major scale.
Chords formed at the perfect fourth interval above the Tonic are as Sub-Dominant chords.
They are the most popular chords for harmony after the Tonic and the Dominant.
We will see this perfect fourth interval later when we examine suspended chords.
Frequency ratio perfect fourth = 4X/3X = 4:3.
There are 6 semitones separating the notes of a perfect fourth interval.