The song to which Klausner referred is 1967’s “Taurus,” by the U.S. band Spirit, and listening to both songs back to back absolutely reveals certain similarities.

The song was written by the group’s guitarist, Randy California, and the suit was brought by one of his trustees, Michael Skidmore. He said that Page may have written the iconic Led Zeppelin song after touring with Spirit in early in their career.

California, who died in 1997, noted the similarity between the two songs in the liner notes to the 1996 reissue of his group’s self-titled debut album.

“People always ask me why ‘Stairway to Heaven’ sounds exactly like ‘Taurus,’ which was released two years earlier,” he wrote, surmising that Led Zeppelin may have first heard the song early in their career when they shared a stage with Spirit. “They opened up for us on their first American tour.”

Speaking with journalist Jeff McLaughlin of Listener magazine, he was more blunt in his assessment.

“I’d say it was a ripoff,” he said. “And the guys made millions of bucks on it and never said ‘Thank you,’ never said, ‘Can we pay you some money for it?’ It’s kind of a sore point with me. Maybe someday their conscience will make them do something about it.”

Lawyers for Led Zeppelin didn’t immediately respond to a request for comment. This post will be updated if they respond. But, previously the Led Zeppelin camp has responded to the suit by saying that as a songwriter-for-hire, California had no copyright claim to the song. Additionally, they said the chord progression at the center of the controversy was so unoriginal that it doesn’t warrant copyright protection.

How much is the song worth?

The value is beyond estimation. SPIN magazine called it “the most popular song of the rock era” and the BBC called it “the most requested song ever played on American radio.” Almost every aspiring guitarist attempts to learn the opening section, which has inspired guitar shops the world over to fine customers who play the song’s opening while trying out an instrument. It’s a backhanded compliment, but it demonstrates how ubiquitous the song has become.

Beyond its cultural significance, there’s also the money. In 2008, Conde Nast Portfolio estimated that the song had earned a figure inching its way towards $600 million. The album on which it appears, “Led Zeppelin IV,” has been certified platinum 23 times over by the Recording Industry Association of America, signifying sales of 23 million copies. Like Pink Floyd’s “Dark Side of the Moon” and “Their Greatest Hits (1971–1975)” by the Eagles, it’s one of those albums that everyone keeps in their collections, and its popularity shows no signs of flagging.

So why now? Why didn’t Spirit file the lawsuit against Led Zeppelin 45 years ago when the song was released, as opposed to waiting until May 2014? According to Rick Skidmore, who has run Randy California’s trust with the guitarist’s mother, it came down to the fact that nobody in the Spirit camp had the money to mount a legal challenge against one of rock & roll’s highest-earning groups, and the phalanx of legal representation that was no doubt available to them.

“I don’t have the resources and barely have the time to do the trust stuff and hold down two jobs,” he told Bloomberg News in 2014.

Previous cases

Led Zeppelin, while denying the validity of the claims in this case, has reached settlements for royalties and songwriting credits with such artists as blues singers Willie Dixon and Howlin’ Wolf, folk singer Anne Bredon and songwriter Jake Holmes. All of them claimed that the rock group had released music that was too similar to their own to be an accident.

Despite the similarity to the Spirit song, there’s no denying that “Stairway to Heaven” remains a beloved musical milestone, one that anyone with access to FM radio is likely to hear any day of the week. Zeppelin guitarist Jimmy Page said as much in a 1975 interview with Rolling Stone.

“’Stairway’ crystallized the essence of the band,” Page told Rolling Stone magazine in 1975. “Every musician wants to do something of lasting quality, something which will hold up for a long time, and I guess we did it with ‘Stairway.’”

Still, Spirit bassist Mark Andes said that his former band mate, Randy California, should be recognized in the credits of future Led Zeppelin releases.

“It would just be nice if the Led Zeppelin guys gave Randy a little nod,” he said. “That would be lovely.”

Daniel Bukszpan is a New York-based freelance writer.