A new trial will be held in a lawsuit that accuses Led Zeppelin of copying a 1960s instrumental in its 1971 classic Stairway To Heaven.
An earlier trial saw a federal court jury in Los Angeles find in favour of Led Zeppelin, but the US appeals court has since ruled that the judge in the original trial had given misleading instructions to jurors.
The instructions were regarding copyright law, which was a vital part of the suit.
The lawsuit was brought in 2015 by Michael Skidmore, a trustee for the late Randy Wolfe, who was guitarist of rock band Spirit and composed an instrumental called Taurus in 1967.
Mr Skidmore says the two bands toured together in 1968 and 1969 and this may have been when Led Zeppelin guitarist Jimmy Page was inspired to write Stairway To Heaven.
According to Mr Skidmore's lawsuit, Wolfe had complained about the similarity between the two songs not long before he drowned in 1997 while attempting to rescue his son.
Plant and Page, however, say that Wolfe was a songwriter for hire and, therefore, had no claim to copyright.
They also say that the chord progressions in Stairway To Heaven were too well-known for such protection.
The judge in the first trial, R Gary Klausner, was found to have failed to tell jurors that, while individual elements of a song such as its notes or scale may not qualify for copyright protection, a combination of those elements may if it is sufficiently original.
Judge Klausner had also wrongly told jurors that chromatic scales, arpeggios and short sequences of three notes were not protected by copyright, the appeals court said.
Circuit Judge Richard Paez said: "This error was not harmless as it undercut testimony by Skidmore's expert that Led Zeppelin copied a chromatic scale that had been used in an original manner."
Also, jurors could only listen to experts' versions of the sheet music for Taurus – not the version of the song recorded by Spirit.
Steven Weinberg, a copyright lawyer who watched the trial, said the sheet music was not faithful to the recording, so jurors could not fairly compare the songs.
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Mr Weinberg said allowing the jury to hear the song "has the potential of changing the outcome at the next trial because the jury will finally get to compare apples to apples".
Francis Malofiy, an attorney for Mr Skidmore, said in a statement that his client had faced "unfair rulings at the trial court level" and looked forward "to the challenge of a fair fight".