He insisted that he did not borrow from “Let’s Get It On,” and, playing an acoustic guitar in the witness box, he showed that the chord progression at the core of both tunes, while similar, was not identical.

In “Thinking Out Loud,” he testified, the second of the four chords in the progression was major, rejecting a suggestion by Alexander Stewart, a musicologist from the University of Vermont hired by the plaintiffs, that it was similar to a minor one in “Let’s Get It On.”

“I know what I’m playing on guitar,” Mr. Sheeran said. “It’s me playing the chords.”

Mr. Sheeran also derided Mr. Stewart’s analysis of his vocal melodies — which included an example with some notes changed — as “criminal.”

A quirk of copyright law governed how the jury could hear the two songs. The case involved only the compositions underlying both tracks — the lyrics, melodies and chords that can be notated on paper — and not their recordings. For older songs like “Let’s Get It On,” copyright is limited to the sheet music, or “deposit copy,” that was originally submitted to the United States Copyright Office. On “Let’s Get It On,” that notation was skeletal.

That meant that the jury never heard Gaye’s original recording, which went to No. 1 in 1973. Instead, the defendants supplied a computer-generated re-creation of what appears on the deposit copy, complete with a robotic voice rendering lyrics like “If you feel like I feel, baby, then come on, let’s get it on.” The studio recording of Mr. Sheeran’s song was heard multiple times.

Kathryn Griffin Townsend, Mr. Townsend’s daughter, who wore a tan-colored coat with the word “integrity” emblazoned on the back, said she had filed her suit to protect her father’s legacy. On Day 3 of the trial, the proceedings were interrupted when she collapsed and was taken to a hospital. When Ms. Townsend returned to the courtroom on Monday morning, Mr. Sheeran embraced her.