This morning, I again listened to both the introduction of Zeppelin's Stairway to Heaven and Spirit's Tarsus instrumental (links to both at the bottom of this post). While there are similarities, there are differences as well. As one commentary to my original post put it, the Spirit instrumental is a standard chord progression whereas the introduction to Stairway to Heaven is much more than that.
The group, Axis of Awesome, very creatively demonstrated that a series of four chords makes up hundreds of popular songs, tens of which are featured in their song "Four Chords" (link below). Yet most would agree that each of the songs featured are themselves unique creative works.
Previously reported in my 2014 post entitled, “Stairway to Copyright Infringement”- Step One in Spirit's uphill climb to prove Led Zeppelin's 'Stairway to Heaven' infringes its copyright", Led Zeppelin was sued by the band Spirit for copyright infringement based on Zeppelin's immortal song which Spirit alleges lifted portions from its guitar instrumental, Taurus. Spirit has now advanced past step two, convincing the judge that there is sufficient evidence to conduct a trial for copyright infringement.
Turning to substance of the debate, does "Stairway to Heaven" infringe Spirit's Taurus guitar instrumental? In order to infringe an original work's copyright, the alleged infringing work must be deemed "substantially similar" to the original work and the alleged infringer has to have had "access" to the original work. In this case, "access" would not appear to be an issue as Led Zeppelin and Spirit toured together in 1968 and 1969.
Below are links to YouTube clips from Spirit's "Taurus" and Led Zeppelin's "Stairway to Heaven." See if you think that the opening notes in Stairway to Heaven are substantially similar to the middle guitar instrumental in Spirit's "Taurus" (e.g. starting around 0:43). Since most are familiar with the iconic guitar progression in "Stairway to Heaven," I present Taurus first. Please let OP-IP know what you think.