Understand music by understanding chord progressions!
Popular music, especially composed on the guitar, is mainly practiced by musicians who are not big on music theory.
Instead the approach is often, if it sounds cool it’s right.
Because of this we have a musical culture that, through evolution, has created variations to a chord progression we today experience as normal.
In beginner chord progression we learned about diatonic chord progressions (chords built off the major scale), this is here at intermediate completed by studying the variations.
It would be fair to say that a combination of diatonic progressions and the most common variations like IVm, IIIx and bVII is what makes up chord progressions of popular music.
The Guitar Conspiracy examine this in much greater detail.
As well as these there is another, very common variation and that is the variation that comes from The Blues.


The variations are as common as the diatonic chords!