Don't know if this has been covered elsewhere....
I haven't been following this closely, but, apparently, in 2014, the successor in interest to a guy named Randy Wolfe sued members of Led Zeppelin and various Warner Music entities. The plaintiff claimed that Stairway to Heaven infringed a song called Taurus, which was performed by a band called Spirit. More specifically, the plaintiff claimed that the opening passage of Stairway infringed the eight measure passage at the beginning of the deposit copy of Taurus (written sheet music).**
After granting a motion for partial summary judgment by the defendants, the court held a five-day jury trial, which resulted in a defense verdict. The jury determined that the songs were not substantially similar (which is the standard for copyright infringement). In 2018, a panel of the Ninth Circuit Court of Appeals reversed the judgment and remanded the case for a new trial. Last week, the Ninth Circuit, en banc (i.e., all of the judges on the court as opposed to the normal three judge panel) reinstated the jury verdict.
https://cdn.ca9.uscourts.gov/datastore/opinions/2020/03/09/16-56057.pdfOne of the more significant aspects of this ruling is that it gets rid of a rule used in the Ninth Circuit (and I believe one other circuit) called the "inverse ratio rule." That rule essentially made it easier to prove substantial similarity for works to which the defendant had a "high degree of access." In other words, where a work is fairly well-known, it set the similarity bar lower than it might be for an obscure or unknown work.
** - Under the 1909 Copyright Act, which was in effect at the time both Taurus and Stairway were created (and which, as of 1/1/1978, was replaced by the 1976 Copyright Act), registration was a prerequisite to copyright, so the court held that only the deposited sheet music was relevant.