[RockofStrength]

I consider the fundamental chords in progressions to be tetrads (seventh and sixth chords). Triads are nice in their ambiguity as to where they are going (and their sonic simplicity), and when a seventh or a sixth note is added, it generally tells you where the chord is headed.

EGs: I I I6 V V V V7 I vs. I I I Imaj7 IV IV IV IV6 I

Sixth chords move up a fifth, seventh chords move down a fifth. (the seventh of a chord falls, the sixth rises - one can also view the IV7 chord in blues music that moves to the I to be a IV+6)

The fundamental tetrad concept introduces the idea of "homophonic chords" - for example, a major sixth chord and a minor seventh chord have the exact same pattern, and the designation depends on context. Often, two designations exist in a superposition until it is settled later on (or never).

Also interesting is chordal metamorphosis, where for example a subtle shift in notes can modulate you into a new mode -for example, V morphing into III7 by chromatically altering a note, taking you from major mode to relative minor. (and this particular V III7 change is a nice way to make a strong chord change when stuck on an already strong chord) EG: Hallelujah - |I |IV V |vi |IV |V |III7 |vi |

Music theory is a very interesting topic, and I've arrived at models that do a pretty good job of explaining the underworkings, allowing me to appreciate the beauty of songs analytically as well as emotionally.