From what I understand, it's the intro to Stairway that's in dispute, which is just a Line Cliche, used in tons of popular songs. According to Forbes, the likely conclusion will be a settlement, which would be unfortunate.

http://www.forbes.com/sites/oliverherzfeld/2014/05/21/spirit-v-led-zeppelin-analysis-of-the-stairway-to-heaven-infringement-lawsuit/

I believe that musicians will benefit from this being argued in the courts. Is it just me, or am I being overly optimistic that the Minor Cliche cannot be copyrighted? How can we get Led Zeppelin to fight this?

I like to see them take it to court.

I have always been under the impression that chord progressions are not copyrightable. After doing a bit of google research I've found that that isn't exactly true, but in order to successfully copyright a chord progression you'd have to come up with an entirely new, unique progression. That absolutely is not the case here. The chords in question here ( Amin, Amin/maj7, amin7) are so common that, as JP noted, they are called a line CLICHE!  If this case was being judged on that basis alone, Randy California, author of "Taurus" would be vulnerable to copyright lawsuits from countless other writers. 

Unfortunately for Led Zeppelin it appears that, according to the article JP posted above, this case would probably be decided, not on expert testimony, (a musician explaining chord theory to a jury) but on whether the two songs are substantially similar to the ears of ordinary members of the jury.  It probably won't help Zeppelin that they have already been to court several times over similar allegations.

 IMO, it's quite possible that it was lifted considering the timing and tour exposure, but so what? Is there "really" anything "original"? I don't agree with the majority of people who support intellectual property rights. Definitely not the same exact song per se, but that descending omnibus progression is there, no question about that.

Intellectual Property (IP) is a complicated issue. Specific to art/music, I like how it protects original works of art from being ripped off. But vulchers will always circle when opportunity arrives. The recent supreme court decision that provides additional time to bring a copyright law suit (http://www.forbes.com/sites/danielfisher/2014/05/19/unusual-split-as-supreme-court-upholds-raging-bull-suit-vs-mgm/) is the latest rot that the vulchers have found, and is the catalyst of this this suit.

The question is, should chord progressions be protected? What about Hang Up Your Hang Ups by Herbie Hancock http://go-ml.com/SpNaM6. The chord progression is very unique and I might argue yes if taken as a whole. What about parts of the entire progression? It depends on how much. And what about Watermellon Man? http://go-ml.com/SpNsCz The chords are simple, so in its entirety, probably not. But the sounds and melodies are unique and represent an original piece, especially the head.

Like I said, its complicated. How much of a chord progression (if any) should be protected? And what about Line Cliche? It would be silly to protect this!

Thieves! Why should this time be any different?

https://www.musiclessons.com/youtube/share/?v=JyvLsutfI5M,zThdTAWQFAQ&t=Plagiarism%20By%20Led%20Zeppelin%3F%20No%20Way%20Dude%21

it seems the piece is a public domain written in the Baroque era by harpist O'Carolan 

https://musictales.club/article/celtic-baroque-roots-stairway-heaven