Robert Plant, lead singer of Led Zeppelin, performing at a concert in MoroccoYoussef Boudlal / Reuters

“It don’t matter what you do, / Hey, hey it’s all up to you.”

You have to be of a certain age to appreciate the irony in this permissive paean, sung in 1977 by Randy California (né Wolfe), the guitarist of the rock band Spirit. Earlier this year, a trustee for the late Wolfe’s estate implied that it matters very much what you do, attempting to sue the rock legends Led Zeppelin for stealing Randy’s music. The lawsuit, heard in June, alleged that Zeppelin’s Jimmy Page lifted the famous fingerpicked acoustic opening of “Stairway to Heaven” (1971) from Spirit’s 1968 instrumental “Taurus.”

Led Zeppelin are no strangers to such accusations. Their iconic “Whole Lotta Love,” from their 1969 album Led Zeppelin II, brought a whole lotta trouble when the blues artist Willie Dixon realized that it used some of the lyrics from his 1962 song “You Need Love.” On the same album, “Bring It On Home” borrowed from another of Dixon’s songs (this time with an identical title), while “The Lemon Song” was partly derived from Howlin’ Wolf’s “Killing Floor.”

This doesn’t exhaust the accusations of musical theft that Page and songwriting partner Robert Plant have faced. Part of the problem, says the music psychologist Richard Hass, of Philadelphia University, was that, particularly in their early days, Led Zeppelin had the habit, time-honored in blues music, of jamming around a known tune to construct new songs. Maybe they just got a bit lazy about hiding the origins, Hass says. Their main problem, though, was probably making enough money that a plagiarism lawsuit becomes worthwhile: “Stairway to Heaven” is estimated to have earned Page and Plant almost $60 million.

It looks all the worse to see a bunch of white guys getting rich by drawing on the works of lesser known and sometimes impoverished black artists—unfortunately another time-honored tradition in popular music. Led Zeppelin reached an out-of-court settlement for the borrowings from Dixon in “Whole Lotta Love,” as well as lucrative settlements with a branch of Chess Records for the other legal suits brought in response to Led Zeppelin II.

But the latest case concerning “Stairway” was dismissed by the court. While Page testified that the band used to jam to another of Spirit’s songs from their eponymous 1968 debut album, he denied ever having heard “Taurus” until suggestions of the similarities began to appear online. You can judge for yourself from several juxtapositions on Youtube. To my ear, there’s certainly a similarity to the musical fragments.

But is similarity enough to qualify as plagiarism? The question is more complicated than it might seem.

You might imagine the issues aren’t so different from those in other artistic fields, especially literature. Charges of stealing someone’s ideas, as the historians Michael Baigent and Richard Leigh claimed of Dan Brown’s The Da Vinci Code, are murky. But if you can put the words side by side and show the overlap, as happened for allegations against How Opal Mehta Got Kissed, Got Wild, and Got a Life, Kaavya Viswanathan’s much-hyped 2006 debut novel, the case is pretty clear. So surely you can do the same comparison with notes and chords, right?

But in music it’s not that simple. The stringing together of notes and chords is bound by rules far more constraining than those that govern linguistic grammar and semantics. In pretty much all of Western music outside of the 20th-century avant-garde, only a limited number of sequences of tones and chords sound “right,” while others seem “wrong.” So it’s very likely that, if you come up with a sequence of chords that sounds good, someone else will have already used it.

Crudely speaking, two chords fit together well if they—or the scales on which they’re based—share many notes in common. Countless songs use a shift from C major to A minor, for instance (think of the first lines of Leonard Cohen’s “Hallelujah”), because the respective scales are closely related. The relationships between chords can be represented as a kind of spatial map of “harmonic space,” showing which are close together and which are far apart. Typically, Western tonal music uses chord sequences that make only small steps in this space from one chord to the next. The paths are very limited, especially in the strongly formulaic and melodic forms of pop and rock. So expecting songwriters to avoid all echoes of other songs is a little like putting someone inside a maze and telling them to find an original way out.

One of the key defenses in the “Stairway to Heaven” case was that Page’s opening chord structure—a “descending minor chromatic progression”—is very old, dating back at least to the baroque era. (It also appears, Page attested, in “Chim Chim Cher-ee,” from the movie Mary Poppins.) And the chord progression in the song’s raucous finale, A minor to F major via G major, is a textbook example of harmonic proximity that has appeared in countless other rock and pop songs, including Bob Dylan’s “All Along the Watchtower.”

This “harmonic map” of chord relationships becomes so mentally ingrained as we listen to music that, at least for musically experienced listeners, it gets literally imprinted in the neurons of the part of the brain (within the prefrontal cortex) that processes harmony. No wonder we stick so closely to its pathways.

Melodies, too—the tune of a song—are highly rule-bound. Melodies in pop, classical, and non-Western music all tend to use predominantly small steps between the pitches of successive notes. They also tend to rise and then descend again, in a kind of arc. At the simplest level, this gives us the rather banal melodic trajectories of nursery rhymes. Pop musicians are often scarcely much more inventive: It always seemed to me that the tunes crooned by Morrissey, of the Smiths, do not dare to venture beyond about a three-note range. But the truth is that, if you want to pen a hummable melody, there aren’t many options in any case.

Purely in statistical terms, then, you’d expect that the rules of “pleasant” harmony and melody will end up generating rather homogeneous bodies of music. Pop and rock mostly juggle with an extremely small range of artistic choices, and often genius and innovation is a matter of putting old wine in new bottles.

Now add to this the possibility of unconscious influence. As a musician you’re listening all the time to others, absorbing, imitating, and reconstituting what they’ve done. Is it so surprising that occasionally you find you’ve more or less just recycled someone else’s song? That’s what George Harrison was deemed guilty of doing in the notoriously protracted 1971 court case that found his “My Sweet Lord” (1970) to be unacceptably close to the Chiffons’ 1963 song “He’s So Fine.” Even the judge decided that this particular case of plagiarism was unconscious, but Harrison still had to pay up.

It’s precisely for this reason that many examples of pop-music plagiarism are merely testaments to our litigious society. The cases are brought because there’s money to be made (especially by lawyers), not because anyone has done anything beyond the pale. Sure, artists deserve property rights like the rest of us, and if your song is blatantly stolen for profit then you deserve compensation. But given the narrow constraints of harmonic and melodic composition, “stolen” in the context of music plagiarism ought to mean more than “sounds like.”

What's more, such charges may also neglect basic truths about what it takes to write a popular piece of music. In short, no one gets famous purely because of their musical originality, and often fame depends on the relative lack of it. In the 1980s, the psychologist Dean Keith Simonton, of the University of California, Davis, measured the “originality” of the melodic themes of several famous classical composers. He analyzed the pitch steps between successive pairs of notes for just the first six notes of their melodies, and compared these against the statistical averages for all such themes in the corresponding era. The greater the departure from the average, the more original the fragment of tune was deemed to be.

Simonton found that the popularity of a composition—judged from how often the works have been performed—initially increased with this measure of originality, but then did the opposite. People like a touch of freshness, but are less keen on melodies that depart too much from what they’ve heard before.

Simonton’s work has recently been extended by Richard Hass, who used more sophisticated methods for measuring melodic originality, which take into account factors such as shifting overall trends in compositional style over time (the influence of the “Zeitgeist,” you might say). Applying these to 553 American songs written between 1900 and 1960—the era often said to have produced the Great American Songbook—he found that the relationship between fame and originality is rather complicated and genre-dependent. All the same, the data confirmed the notion that if you’re too original, you’ll probably be writing for a rather small audience. “In some ways,” Hass says, “to be successful as a songwriter, you have to utilize elements of existing music.”

Experimentation is essential; music would ossify without it. But commercial success inevitably rides on the back of others’ ideas. And there’s no shame in that. It’s not to deny any of the genius of the Beatles to say that they could never have written their songs without hundreds of years of musical tradition from which to draw. Which makes it all the more ironic that their estates guard the copyrights to these songs so jealously. The lawyers wouldn’t even sanction the use of “Here Comes the Sun,” written by George Harrison, on the golden record of humankind’s musical oeuvre sent by NASA into the cosmos aboard the Voyager spacecraft in the 1970s.

It was partly in recognition of the necessarily formula-bound character of tonal music that the notion of plagiarism—or at least condemnation of it—once would have seemed absurd to composers. In the 17th and 18th centuries, such borrowing was called “parody” (without the derogatory connotation it has today). Johann Sebastian Bach, for example, helped himself promiscuously and un-self-consciously from both his own earlier works and those of others. According to the 19th-century German music historian Philipp Spitta, Bach felt his appropriation of secular music in his sacred cantata was not only justified but proper: These borrowings, Spitta wrote in Johann Sebastian Bach, “were made with a perfect feeling that the compositions in question would not be seen in their right place till they were set for Church use.” If Bach used reused familiar hymns, meanwhile, that just helped the music resonate with the congregation.

This kind of recycling acknowledged implicitly that it’s not just about where you get your ideas from. Music is inherently referential: Much of what it says to us relies on quotation, whether that’s Béla Bartók giving voice to his native Hungary by rearranging folk songs or the Jam nicking a bassline from the Beatles’ “Taxman” for their 1980 song “Start!”   

The problem, then, is how to respect this character in an age when music has also become a big business, with all the issues of ownership and property rights that entails. The more proprietorial music gets, the further it recedes from what it is about in the first place: a communal human enterprise, arguably the finest one we’ve ever invented. It’s a magpie tradition, and that’s a big part of why it matters.

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