I - V/3rd - VIm - IV: pop/rock ballads width descending bassline

A little change in just one chord of the I V VIm IV progression leads us to the typical sound of rock and pop ballads that you know from countless hit songs.

In the major chord on the 5th scale degree we play the 3rd as the lowest note instead of the root. Like this a G major chord for example becomes a G/B chord (say "G over B").

In this video I'm playing the progression through all 12 keys (with just some of the chord variations shown above):