Audio: pop modulation (0:16)
pop modulation is a power trio playing modulating chord progressions. The pop modulation figure shows the score.
Modulation is one of the two main ways to vary a harmony (the other is to expand a chord into a chord progression). It is an excellent way to differentiate the sections of a pop song from each other.
There are no rules for modulation in pop harmony. The harmony can modulate to and from any key. It can be smooth and gradual, akin to a cadence in functional modulation, or direct and abrupt. Modulation can be short-term tonicisation, lasting a single bar, or for longer. The principles of modulation in pop harmony and the types of modulation are the same as for melody, and you can modulate freely between relative keys, close keys, parallel keys and distant keys.
pop modulation demonstrates another approach to harmonic modulation. It takes the three chord trick I-IV-V, keeps the chords the same, but modulates from I to the key of V and then to the key of IV. Simple and effective. The bonus is two new chords.
Three chords form the C-F-G-F progression, I-IV-V-IV in the key of C, in bars 1-2 of pop modulation. Bars 3-4 modulate to the key of G. The same three chords are used in the progression G-G-F-C, but the progression is I-I-bVII-IV in the key of G. Bars 5-6 do the same but this time in the key of F where the progression F-G-C-F is notated I-II-V-I. The two new chords are a flat seven, bVII, and a major second, II. This is no coincidence either. If you look at the circle of fifths, you will find that the two most closely related keys to I, after IV and V, are bVII and II.
The flat seven chord, bVII, is a cracker. It is a staple of pop harmony. So much so that pop musicians, especially rock guitarists, are born with a special gene that enables them to play this chord from birth. The chord progression I-bVII-IV-I is an interesting and versatile chord progression. The root moves down two consecutive fifths, a step, from I to bVII, then up a fifth to IV, then up another fifth back to I. Root progression by fifth is the strongest movement of all and explains why the progression sounds so effective.
The major second chord, II, is another excellent chord. I-II-V, with its major second, occurs in functional modulation as a way to modulate from the key of I to the key of V. In pop harmony, the II chord exists as a nonfunctional chord in its own right. Bars 5-6 include the major second in the F-G-C-F progression, I-II-V-I in the key of F. Bars 7-8 also include a major second in the arguably stronger sounding I-II-IV-I progression, G-A-C-G in the key of G.
There are no key signature changes in the pop modulation score. It is a matter of taste whether to include them or not. They have been omitted so as not to clutter the score with key signatures and accidentals and to make the different relationships between the chords in the three chord trick more obvious.
Transposition is another modulation technique that is used in pop harmony. You could argue that bars 7-8 are the same as bars 5-6 in pop modulation with the key transposed up a step from F to G and the I-II-V-I progression tweaked to make it I-II-IV-I. For some obscure reason, this type of transposition up a step is known as the Truck Driver's Gear Change.