NEW YORK, NY.- For most music fans, a song is a simple thing to define: It’s the melodies, the lyrics, the grooves that come out of your speakers.

It’s a much thornier question when it comes to copyright law, one that has been tested in a series of high-profile lawsuits over the past decade, involving stars like Ed Sheeran, Led Zeppelin, Pharrell and Robin Thicke. Is songwriting defined by what you hear on a recording, or the notes inked long ago on a piece of sheet music? Where does a composer’s work end, and a performing artist’s begin?

In other words, what, exactly, is a song, in the eyes of the law?

In many music copyright disputes, one of the main issues is originality, or how the law sets a boundary between creative expression that is the property of a single artist versus material in the public domain. Last year, a federal jury in New York heard hours of expert testimony about whether a syncopated four-chord sequence in Marvin Gaye’s “Let’s Get It On” was distinctive enough that Sheeran’s song “Thinking Out Loud” infringed on it — or whether, as Sheeran’s lawyers contended, those parts are generic “building blocks” that no musician can own. The jury ruled in Sheeran’s favor, finding that he and a co-writer had created their song independently and not copied from Gaye’s 1973 classic.

But a key question running through that trial was about something even more fundamental: whether the core of “Let’s Get It On” — and what is protected by its copyright — is determined by the sounds we hear on its original recording, or the notes written on yellowing sheet music stored at the Library of Congress.

That issue was at the center of an appeals court’s decision four years ago regarding Led Zeppelin’s “Stairway to Heaven,” and it is being considered in another appeal related to Sheeran and “Let’s Get It On.” Many experts believe it’s an underexplored question that gets to the heart of how copyright law intersects with music.

“This is the deep, existential, metaphysical question at the center of music copyright: We don’t even know what it is,” said Jennifer Jenkins, a law professor at Duke.

It is also an important question for an industry that in recent years has poured billions of dollars into deals for song catalogs, partly on the faith that their underlying copyrights offer robust protection against infringement. That may be challenging for older songs, because of a quirk in the law that can restrict how a song is defined and, therefore, just what its author owns.

How does a song get copyright protection?

There is a key date related to this issue: Jan. 1, 1978, when the last major revision of United States copyright law took effect.

Since then, songwriters have been able to register a composition with the Copyright Office by submitting a recording; all the melodies, chords and lyrics on it are considered evidence of their work. But earlier songs were subject to the Copyright Act of 1909, which required that songwriters submit transcribed sheet music, known as deposit copies.

For a century, these deposit copies were little more than receipts in a copyright paper trail. But since the “Blurred Lines” case a decade ago, when Pharrell and Thicke were found to have copied Gaye’s “Got to Give It Up” and ordered to pay more than $5 million in damages, these once-obscure documents have taken on a greater significance.

The judge in that case ruled that, under the 1909 law, the deposit copy for “Got to Give It Up” (1977) determined the “scope” of that song’s copyright. In other words, only the notes on its paper submission counted as representing Gaye’s songwriting creation, and any other elements that were on the song’s recording but not the deposit copy — like percussion and studio atmospherics — were not part of the underlying composition. (A separate copyright applies to the recording.) The jury, instructed to consider only what was on Gaye’s deposit copy, found that “Blurred Lines” had copied from it.

These deposit copies can be minimal, sometimes featuring just a vocal melody and indications of chords. The one for “Taurus,” a 1967 piece by the psychedelic rock band Spirit, which Led Zeppelin was accused of copying on “Stairway to Heaven,” is a single-page sketch of barely 100 notes, and a lawyer representing Michael Skidmore, a trustee of the trust that owns rights to “Taurus,” argued that it was not even an accurate transcription.

The deposit copy for “Let’s Get It On” is five pages but omits elements like piano, drums and guitar — including the wah-wah opening guitar lick that has been a Pavlovian call to the dance floor at many a wedding — that are part of the signature sound of Gaye’s original track.

The judge overseeing Sheeran’s trial, citing an appeals court’s detailed decision in the Led Zeppelin case, ruled that the absence of those elements from the “Let’s Get It On” deposit copy meant that lawyers for the plaintiffs — family members of Ed Townsend, Gaye’s co-writer and producer — had to restrict their arguments to the vocal melody and the chord pattern. That restriction likewise applied to Alexander Stewart, a music professor at the University of Vermont, who testified as an expert witness for the plaintiffs.

“Every time I opened my mouth and said the word ‘bass line,’ I was cut off,” Stewart said in an interview. “It was hard to make any cogent argument.”

Following a ruling by the judge, Gaye’s recording was never played for jurors. Instead, they heard an electronic realization of the deposit copy, submitted by the defense. It included bare piano chords and a robotic-sounding vocal — an oddly cold interpretation of one of pop music’s supreme erotic anthems. As the track played, quizzical expressions came over a few jurors’ faces.

Some scholars say the legal distinction between a composition in a deposit copy and what appears in a finished recording is a sign that the law has not kept up with how pop music has been made for decades. Very often, songs are created in the recording studio, and the line between composing, producing and performing can be fuzzy.

“It is completely divorced from actual music-making practice,” said Joseph P. Fishman, a professor at Vanderbilt Law School.

Jenkins, of Duke, said these cases point to one of the basic complexities of applying copyright — a concept originally made for books and other written material — to music.

“Music is first and foremost an auditory art form, but for most of copyright’s history it’s been defined as something you see,” Jenkins said. “There’s this disconnect, where the signifier — that written thing — is what a composer owns, but the signified is what the song actually is. It’s what we’re listening to.”

Is a whole song on the sheet music?

Exactly why deposit copies have gone from obscure legal formalities to hot topics in some of the biggest music lawsuits of the last decade is unclear. One theory is that historically, most accusations of infringement have involved the most prominent elements of a song, like the hook, vocal melody or lyrics — things that even the plainest sheet music would highlight.

But as pop music has evolved, and techniques like sampling have become standard, it has also become more common for background elements and secondary parts to be in dispute. Many recent cases, like those over “Blurred Lines” and Katy Perry’s “Dark Horse,” have focused on these aspects. (To some observers, the “Blurred Lines” verdict seemed to give the Gaye estate control over a loose rhythmic groove, though lawyers argued that it involved specific notes on a deposit copy.)

David Pullman, an investor whose company Structured Asset Sales is bringing the Sheeran appeal — he is best known for creating “Bowie bonds,” backed by David Bowie’s music royalties, in the 1990s — said he believes that many current artists borrow too much from popular old songs. “It’s easier to take a shortcut and infringe,” Pullman said in an interview, “than write a song that’s original.”

The history of the Sheeran case, now before the 2nd U.S. Circuit Court of Appeals, is complex. Structured Asset Sales, which owns an 11.11% interest in “Let’s Get It On,” filed its own suit over “Thinking Out Loud” after a judge blocked the company from joining the Townsend family’s original action.

Structured Asset Sales’ suit was dismissed by a district court judge shortly after Sheeran won at trial last year. In its appeal, the company argues that a deposit copy does not necessarily define the scope of a song’s copyright under the 1909 law, and that the material on the sheet music only needs to be sufficient to identify it, despite the law’s reference to a “complete” copy. In court papers, Hillel I. Parness, a lawyer for the company, argued that expert witnesses should be able to interpret deposit copies for the jury, as happened at a trial involving singer Michael Bolton in 1994.

Sheeran’s lawyers argue that the deposit copy rule is clear from the law, and is supported by long-standing guidance from the Copyright Office. At oral arguments last month, Donald S. Zakarin, a lawyer for Sheeran, also warned that straying from a deposit copy’s notation could lead to problems of “subjectivity” when defining a musical work that is in dispute.

“Future authors,” Zakarin said, “are going to be subjected to, ‘No, no, no, I intended to have that bass line. I know it’s not there, but I intended it.’”

In an informal survey of about a dozen intellectual-property experts, most said Sheeran’s side had the stronger argument about deposit copies under the 1909 law. “It’s a lousy rule,” said Fishman. “But that does seem to have been the rule at the time.”

But this rule, as set down in the Skidmore v. Led Zeppelin appeal, has at least one prominent skeptic in Paul Goldstein, a professor at Stanford Law School who is the author of a widely cited copyright treatise.

“Where I disagree with Skidmore,” Goldstein wrote in an email, “is in its assertion that the fact the copyright was secured by deposit of a copy of the musical work implies that the deposit copy defines the scope of copyright in the work to the exclusion of any other relevant evidence.” That evidence, he said, could include things like drafts and correspondence around a song’s creation.

Goldstein pointed to another possible source of evidence: sound recordings submitted to the Copyright Office as a supplemental registration. Under that theory, a songwriter with a deficient deposit copy of an old song could, since 1978, submit a recording of it to cover any additional elements — bass lines or guitar solos, for example — absent from the original registration.

This workaround was suggested by the Copyright Office and the Justice Department in an amicus brief filed in the Led Zeppelin appeal. It was apparently little known at the time, though Structured Asset Sales’ court papers note that in 1988, the music publisher for the Rolling Stones’ song “Sympathy for the Devil” submitted that track’s 1968 studio recording to cover a new “arrangement.”

Pullman, of Structured Asset Sales, said that discussion of this workaround during the Led Zeppelin appeal led him to submit the recording of “Let’s Get It On” as a new registration in 2020, to cover any compositional elements not on the deposit copy — which could be more ammunition in a dispute against Sheeran’s “Thinking Out Loud.”

Could it work? So far that issue has not been tested by the courts.

This article originally appeared in The New York Times.