Axis of four awesome chords

[ALBINOGEK]

What are they & how do they relate to each other?

[JonPR]

I-V-vi-IV.
Eg C-G-Am-F, in key of C.
G-D-Em-C in key of G.

[Scotch]

It's the most popular chord progression. For that song its in the Key of E so will go like E, B, C#Minor, A.

Then it shows with a simple change of strumming pattern/ rhythem it turns into another song.

[JonPR]

It is, of course, a variation on what was the most popular chord progression in the 1950s/early 60s: I-vi-IV-V (C-Am-F-G).