Significance of pop songs with progressions that alternate one chord with one that's three semitones below it?

Question

I've noticed a couple of pop songs that use one chord progression that alternates between swapping one chord (either fifth or fourth) with one that's three semitones below it (to third or second): * [Bob Dylan - Knockin' on Heaven's Door](https://www.youtube.com/watch?v=_bWzyiU-S_w) alternates between I V IV and I V ii * [Iggy Pop - Passenger](https://www.youtube.com/watch?v=hLhN__oEHaw) alternates between vi IV I V and vi IV I III * [Cat Stevens - Father and Son](https://www.youtube.com/watch?v=b-7c4VNGOgU) - the father's vocals change from I V IV ii7... to I iii7 IV ii7... I was wondering if there was any musical significance behind this. My guess is that I feel the song could be played with one or the other - is the alternation just to add a bit of variety? And if so, is there a name for this technique, and is there any special relationship between the fifth and the third chord, and the fourth and the second chord that makes this possible?

Answer 1

There are three important degrees in tonal music: I, IV & V. Other degrees can be seen as related to these main ones, functionally. For instance, iii and vi are related to I; iii is IM7 without the root, and vi is IM6 without the root. ii is related to IV for the same reason vi is related to I. vii is related to V as the notes of vii are those of V7 without the root. If you use four-note chords, I6 and vi7 are even inversions of each other, and so are IV6 and ii7.

Answer 2

Hi! They're using a different chord with the same harmonic function. It's a basic harmonic resource.

These are chords you can count in for those substitutions you mention:

Interchangeable chords:

TONIC: I, III, VI

SUBDOMINANT: II, IV

DOMINANT: V, VI

Indeed, if you can notice, all chords with the same harmonic functions are related.

If you play a F triad (F, A, C) and then add D for the bass, it's a Dmin7! Those are the SUBDOMINANT chords from the II and IV degrees in the C major scale!

Same thing if you play Bdim (B, D, F) and the add G for the bass... you got G7! These are the DOMINANT chords of the scale!

...and so on. Tonic chords are also related.That's why you can use Am or Em to replace a C chord in a song, yielding a soft minor sound when you use Am (6th degree chord) or a darker vibe (when you use Em, the III degree).

Have fun!

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Cheers