The heirs of Ed Townsend, Gaye’s co-writer of the 1973 soul classic, sued Sheeran, alleging the English pop star’s hit 2014 tune has “striking similarities” to “Let’s Get It On” and “overt common elements” that violate their copyright.

The lawsuit filed in 2017 has finally made it to a trial that is expected to last a week in the Manhattan federal courtroom of 95-year-old Judge Louis L. Stanton.

Sheeran, 32, is among the witnesses expected to testify.

“Let’s Get It On” is the quintessential, sexy slow jam that’s been heard in countless films and commercials and garnered hundreds of millions of streams, spins and radio plays over the past 50 years. “Thinking Out Loud,” which won a Grammy for song of the year, is a much more marital take on love and sex.

While the jury will hear the recordings of both songs, probably many times, their lyrics — and vibes — are legally insignificant. Jurors are supposed to only consider the raw elements of melody, harmony and rhythm that make up the composition of “Let’s Get It On,” as documented on sheet music filed with the United States Patent and Trademark Office.

Sheeran’s attorneys have said the songs’ undeniable structural symmetry points only to the foundations of popular music.

“The two songs share versions of a similar and unprotectable chord progression that was freely available to all songwriters,” they said in a court filing.

Townsend family attorneys pointed out in the lawsuit that artists including Boyz II Men have performed seamless mashups of the two songs, and that even Sheeran himself has segued into “Let’s Get It On” during live performances of “Thinking Out Loud.”

They sought to play a potentially damning YouTube video of one such Sheeran performance for the jury at trial. Stanton denied their motion to include it, but said he would reconsider it after he sees other evidence that’s presented.

Gaye’s estate is not involved in the case, though it will inevitably have echoes of their successful lawsuit against Robin Thicke, Pharrell Williams and T.I. over the resemblance of their 2013 hit “Blurred Lines” to Gaye’s 1977 “Got to Give it Up.”

A jury awarded Gaye’s heirs $7.4 million at trial — later trimmed by a judge to $5.3 million — making it among the most significant copyright cases in recent decades.

Sheeran’s label Atlantic Records and Sony/ATV Music Publishing are also named as defendants in the “Thinking Out Loud” lawsuit. Generally, plaintiffs in copyright lawsuits cast a wide net in naming defendants, though a judge can eliminate any names deemed inappropriate. In this case, however, Sheeran’s co-writer on the song, Amy Wadge, was never named.

Townsend, who also wrote the 1958 R&B doo-wop hit “For Your Love,” was a singer, songwriter and lawyer. He died in 2003. Kathryn Townsend Griffin, his daughter, is the plaintiff leading the lawsuit.

Already a Motown superstar in the 1960s before his more adult 1970s output made him a generational musical giant, Gaye was killed in 1984 at age 44, shot by his father as he tried to intervene in a fight between his parents.

Major artists are often hit with lawsuits alleging song-stealing, but nearly all settle before trial — as Taylor Swift recently did over “Shake it Off,” ending a lawsuit that lasted years longer and came closer to trial than most other cases.

But Sheeran — whose musical style drawing from classic soul, pop and R&B has made him a target for copyright lawsuits — has shown a willingness to go to trial before. A year ago, he won a U.K. copyright battle over his 2017 hit “Shape of You,” then slammed what he described as a “culture” of baseless lawsuits intended to squeeze money out of artists eager to avoid the expense of a trial.

“I feel like claims like this are way too common now and have become a culture where a claim is made with the idea that a settlement will be cheaper than taking it to court, even if there is no basis for the claim,” Sheeran said in a video posted on Twitter after the verdict. “It’s really damaging to the songwriting industry.”

The “Thinking Out Loud” lawsuit also invokes one of the most common tropes in American and British music since the earliest days of rock ‘n’ roll, R&B and hip-hop: a young white artist seemingly appropriating the work of an older Black artist — accusations that were also levied at Elvis Presley and The Beatles, whose music drew on that of Black forerunners.

“Mr. Sheeran blatantly took a Black artist’s music who he doesn’t view as worthy as compensation,” Ben Crump, a civil rights attorney who represents the Townsend family but is not involved in the trial, said at a March 31 news conference.