Does anyone know a textbook or some music theorist explaining subdominant/dominant chord functions in terms of the two chords being the tonics in subdominant/dominant keys?
Background: I was thinking on how a person can reach to an idea similar to tonic/subdominant/dominant (i.e. the three categories of chords which have different degrees of unstableness) just by interval/key relationships, not by historical theories.
My current idea follows like this:
- Harmonic series convince us that major chords are more consonant than minor chords or diminished chords because major chords appear as the 4/5/6th overtones.
- A major diatonic scale (e.g. C major scale) can build three major triads, three minor triads, and one diminished triad. By major triads being more consonant, we can ignore less important parts (minor triads and diminished triad) and are interested in how the remaining three major triads relate to each other.
- Among the three major triads, "triad whose root is the major scale's first note" (e.g. C major chord in the case of C major key) sounds most stable, given that the first note in the major scale is the most stable note. As a result, tonic chord is now considered as the most stable chord.
- Then, we can observe that the remaining two major chords, one starting from the 4th note and the other from the 5th note, are also the tonic chord of other keys (e.g. F major key and G major key). The two chords are "the most stable" in those keys, but because we're currently thinking in a different key (e.g. C major key), they are currently used in an "unstable" way. Like in the middle of modulation or something.
- Since we can't consider the two major chords as having the same quality (one being the right of the tonic chord in the circle of fifth, and the other being the left), we name it differently based on the names of keys: subdominant chord and dominant chord.
The reason I'm thinking about this is because I'm guessing thinking in tonic/subdominant/dominant keys in parallel may lead me understand some non-traditional chord progression, like IV-I-IV-I, better (IV-I-IV-I in tonic key is V-I-V-I in subdominant key) and may work as a more handy way to think when I compose a song.