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

Chords whose roots are a third apart share two notes, and so they sound similar and can fulfill some of the same functions in a harmonic progression. I supposed this is a simple form of chord substitution, although usually that term is reserved for more complex relationships in jazz.