Led Zeppelin's return to court over their most famous song might have sweeping implications for copyright law.

The classic-rock legends have been embroiled in an ongoing legal battle over alleged similarities between the intro to 1971's "Stairway to Heaven" and a late-'60s instrumental called "Taurus" by the band Spirit. After a series of twists and turns along the way, including dramatic in-court testimony by Jimmy Page and Robert Plant, Led Zeppelin appeared to have finally won in 2016. Then a U.S. appeals court suddenly ordered a new trial last year that will focus more specifically on issues surrounding the song's copyrights.

Jurors in the first proceedings also heard from Michael Skidmore, a trustee for the late Spirit songwriter Randy California's estate who filed the lawsuit against Led Zeppelin in 2014, one of California's Spirit bandmates, musicologists and other experts. During the five-week original trialThe Hollywood Reporter said they argued over whether members of Led Zeppelin had heard "Taurus" before sessions took place for Led Zeppelin IV, the commonality of the intro's descending chromatic scale and just how similar the two songs actually are.

Crucially, the original presiding judge ruled that jury members would have to decide based on live performances of the two tracks in court, rather than recorded versions – since pre-1978 copyright laws applied only to sheet music. Back then, songs were registered through paperwork personally deposited at federal offices in Washington, D.C. These so-called "deposit copies" were skeletal handwritten documents created by a label representative who listened to the record.

As such, many of the defining characteristics of these legacy compositions beyond their main melodies – including solos, horn charts, bass lines, background vocals and, yes, intros like the one in "Stairway to Heaven" – were not written down. Perhaps unsurprisingly, when a pianist using the deposit copy as a musical guide performed "Taurus" for the court, it barely sounded like the Spirit song at all, according to the Associated Press. The jury couldn't make the connection based on the music as it was transcribed.

Listen to Spirit's 'Taurus'

A judge later ruled in Led Zeppelin's favor after jurors said they believed Page and Plant had heard "Taurus" prior to creating their song, but couldn't find substantial similarities between the tunes, Fox reported.

Even so, opposing attorney Francis Malofiy's line of questioning on whether "Stairway to Heaven" replicated "Taurus" as it was specifically submitted in paperwork to the U.S. Copyright Office ultimately changed the trajectory of these proceedings. One of Page's answers not only appears to have reignited the case, it might lead to a court decision that forever changes our understanding of the way older songs are protected.

It was an exchange that went largely unremarked upon at the time: Malofiy asked Page to point to protected elements – including that intro – in the "Stairway to Heaven" deposit copy, which was listed as Exhibit 2708. Page was forced to admit that neither the concluding solo nor the disputed intro were there. "That's not represented in the deposit copy?" Malofiy reiterated, to which Page said, "No. You're correct."

So, maybe none of it was copyrighted in the first place? Malofiy decided to appeal, and the 9th U.S. Circuit Court of Appeals in San Francisco agreed that the matter should be looked at more deeply. The only court-requested briefs so far relate to deposit copies, according to a new report by Bloomberg's Vernon Silver.

Listen to Led Zeppelin's 'Stairway to Heaven'

Silver visited the U.S. Copyright Office, where paperwork for songs from before 1978 is still filed in an old-fashioned card-catalog system. What he found was shocking: Deposit copies there did not include transcriptions for the dueling guitar finale on Eagles' "Hotel California," the iconic Clarence Clemons sax solo from Bruce Springsteen's "Born to Run" or Ray Manzarek's rain-like turn on the Fender Rhodes from the Doors' "Riders on the Storm."

Legal experts now disagree on whether those elements are protected. That forced Led Zeppelin's attorneys to change tactics, since this revelation put their client's signature song at risk. They're now arguing in pre-trial court filings that the portions of "Stairway to Heaven" not outlined in the deposit copy "became protected by federal copyright on the earlier of Jan. 1, 1978, or the first public distribution of copyrighted sheet music," according to Bloomberg.

But that's not the way copyright law has been interpreted in the past. If the court agreed, however, such an update would potentially put Led Zeppelin in a no-win situation, since Malofiy believes that also widens protections for Spirit's "Taurus" too.

"They blew their fucking foot off," Malofiy told Silver. "By virtue of their stupid fucking logic, the whole thing of 'Taurus' is afforded protection. I can't believe they fell into that bear trap."

Malofiy still wants to play the actual recordings of "Taurus" and "Stairway to Heaven" in court. In the meantime, an 11-judge appeals-court panel will reconsider the case before trial, Malofiy told Reuters, focusing exclusively on whether to broaden copyright protections.


Led Zeppelin Albums Ranked

Why Led Zeppelin Won’t Reunite Again

More From Ultimate Classic Rock