Hello and welcome to part 3 of this exciting tutorial on Chord Progressions, where we will be talking about Common Tone Substitution. *cue dramatic intro music*
Any harmonized scale contains notes that are shared within other chords found in this scale. This is the basis for common tone substitution. In the major scale, the I, iii and vi chords have 2 notes in common. Since they contain common tones, they all can function as a I chord within a progression.
A chord progression with just the I, iii and vi chords will soon become boring because it has no tension. A progression such as this is good for giving the illusion of harmonic movement.
The ii and IV chords also share common tones which contrast with the I ,iii and vi chords and can either be major or minor, based on the type of progression you are looking to create.
Lastly, the V and vii chords substitute for each other. Either will function as a dominant chord and will usually pull to the IV or I chord because of the tension within these chords.
For part 4, we will be going over Harmonic Rhythm so stay tuned.