It's an alluring notion: that all music is inspired by what went before; that songs are ever-evolving reboots of earlier versions; that, in art, it's OK to pilfer the past. But the Rolling Stones guitarist's romantic viewpoint ignored one thing: United States copyright laws. Over the years these rules have caused musicians untold headaches and cost them millions in payouts. For many songwriters, overzealous borrowing has been the forbidden fruit that has seen them unceremoniously expelled from the Garden of Eden.

The issue of plagiarism will hit the headlines again next month when a US jury decides whether Robert Plant and Jimmy Page of Led Zeppelin stole the opening chords of their 1971 classic Stairway to Heaven. A Los Angeles judge has ruled that the song is similar enough to a 1967 instrumental called Taurus by the band Spirit to warrant a jury trial for copyright infringement. The case has been brought by a trustee of deceased Spirit guitarist Randy Wolfe, also known as Randy California. Led Zeppelin deny the claim.

So where does the line between inspiration and theft lie?

It is a question Led Zeppelin have been grappling with for decades. The credits for several of the band's songs – including Whole Lotta Love and Dazed and Confused – have been altered over the years after successful legal challenges. Francis Malofiy, the US lawyer spearheading the Stairway to Heaven lawsuit, has even gone so far as to call Led Zeppelin "the greatest covers band of all history".

How do such cases – in which a jury of non-music experts make huge decisions – actually work? And in the wake of the high-profile Blurred Lines trial last year, which found musicians Pharrell Williams and Robin Thicke guilty of copying Marvin Gaye's Got To Give It Up, is it now open season on anyone who has written a song that sounds similar to another?

Dr E. Michael Harrington, a musicologist who has been an expert witness in copyright issues involving Lady Gaga and Busta Rhymes, says that for inspiration to become theft, it has to go beyond common musical cliches. Down the phone from Nashville, he plays a medley including Every Breath You Take by the Police, Quit Playing Games (With My Heart) by the Backstreet Boys and Isn't She Lovely by Stevie Wonder. The songs share melodies and scale degrees that are so common they go back to Bach and Mozart, he says. Suing over them would be like "looking at a page of verse and not wanting to see the verb 'to be'."

Richard S. Busch is a lawyer for Marvin Gaye's heirs, and won the family an initial $US7.4 million settlement in the Blurred Lines case (reduced to $US5.3 million, a settlement that Williams and Thicke are contesting). Busch says the line between inspiration and theft is "a matter of degree".

"There is a point at which you're copying not just an era – or you're not just trying to invoke the sound of a particular artist – but you are actually copying the compositional elements, whether it be notes or whether it be the structure. And, in combination, that can cross the line into copyright infringement," he says.

His recent case illustrates this. Blurred Lines and Got To Give It Up plainly share a similar beat and sense of era. But Busch says that Williams and Thicke copied Gaye's bassline, his percussion and certain notes, while both compositions also have "a spoken part that began and ended at the exact precise [same] point". Howard King, lawyer for Williams and Thicke, disagrees. "There is no overlap in Blurred Lines. There are no two notes in the same place of the same duration between those two songs," he says.

Although a jury sided with Gaye, inspiration versus theft can be a tricky distinction to the untrained ear. But the vagaries of what constitutes plagiarism seem straightforward when compared with what actually happens in court.

In California, where most high-profile cases seem to take place, a jury trial will happen only if a judge rules that there are enough compositional similarities between two songs to warrant it. Eight jurors are then required to apply the "ordinary observer" test to the songs – do they, as non-music experts, see similarities? But juries are often comparing apples with pears. This is because songs recorded before 1978 that are alleged to have been ripped off are dealt with under the 1909 copyright act, meaning that only the song's sheet music – rather than the recorded version – can be submitted as evidence.

So in the Blurred Lines case, the jurors had to compare a recording of Williams and Thicke's 2013 song with the sheet music of Got To Give It Up (1977). The only elements of Gaye's recorded song played were those parts that Gaye's team alleged were copied and that the judge also deemed were included in the sheet music. The Stairway case is in the same court.

Confused? Try being a juror. Enter the all-important musicologists. Called as expert witnesses by both sides to try to convince the jury whether plagiarism has or hasn't occurred, musicologists hold the golden key. And they can be paid a small fortune for their services.

Dr Harrington talks me through his opinions regarding Stairway to Heaven. It's a fascinating analysis: a technical unpicking of the two songs involving analysis of chromatic scales and minor chords. At one point he compares a certain passage to Henry Purcell's Dido and Aeneas opera, written 280 years before Led Zeppelin formed.

Trials, therefore, can hinge on how plausible – and, some would argue, how well incentivised – a musicologist is. King claims that the Gaye heirs paid their musicologist $US350,000. Busch says this figure is misleading.

One thing is clear. As a result of the Blurred Lines verdict, there has been a huge rise in plagiarism claims.

Dr Harrington says things have gone "crazy". He is contacted up to 30 times a month by songwriters alleging foul play, twice as often as before Blurred Lines. He says 95 per cent of claims are nonsense and go nowhere. For example, one hopeful chap thought he had a case against Sam Smith simply because his song opens with, but never repeats, the words "stay with me", the name of Smith's global smash. Dr Harrington charges up to $US5000 ($A6919) a day for his services, but takes on only between 10 and 30 cases a year. "There's an old saying: 'Where there's a hit, there's a writ'," he says. "It's funny – but true."

The Sunday Telegraph, London