I don't think this fact changes any of the arguments, but just so you know, a chord progression cannot be copyrighted. Only a melody. Of course if you sample a chord progression you are sampling a recording, but if you write a tune over someone else's chords with a different melody you are fine. Happens all the fucking time. The most common chord progression in pop music (based on my own unscientific research) is I - V - vi - IV and the second most common is vi - IV - I - V, which is exactly the same as the first except it starts on the vi. Just off the top of my head: Closing Time, Building a Mystery, track 2 from Green Day Dookie, that song with the circus freaks in the video by Red Hot Chili Peppers, also Under The Bridge by them, With Or Without You, Save Tonight by Eagle Eye Cherry, the chorus of Take On Me... just imagine all the amazing pop songs we'd lose if they couldn't have been written because of copyright law (I don't know how to do the comic sans thing)