The new stage musical about Louis Armstrong on Broadway opens with the jazz icon in a rehearsal room alongside an anonymous piano player. Keep an eye on the guy on keys.

Audiences may not know it, but there are actually two Armstrongs onstage at that moment — the actor playing the great trumpeter and one of his real-life descendants. In an act of casting kismet, the piano player is Brandon Louis Armstrong, the music giant’s great-great-nephew.

“That moment always feels like I get to — as Brandon — speak to my great-great-uncle from beyond,” he says. “I get to just spend a moment in conversation with him and ask if he’s OK and hear his voice.”

The younger Armstrong is making his Broadway debut in “A Wonderful World: The Louis Armstrong Musical,” playing multiple parts, including a teacher and as an understudy for Louis.

“I was cautious about how I’d be able to step into this world and connect with part of my family history and my lineage,” he says. “I’m so grateful and so thankful and so happy.”

“A Wonderful World” — starring Tony winner James Monroe Iglehart and James T. Lane in the title role — is a stage bio of one of jazz’s most influential figures, a musician known for “Hello, Dolly,” “Cheek to Cheek” and “A Wonderful World.”

The musical looks behind his wide smile to dig into his rise from poverty, his four marriages and battles with racism. He emerges from it a transcendent force, warts and all.

“The messaging behind it is that jazz is the choices that we make in between the notes,” the younger Armstrong says. “It’s not about making the right choice every time. It’s about making a choice and allowing yourself to be present wherever those choices may lead you.”

The younger Armstrong was born and raised in Los Angeles. He graduated from The American Musical and Dramatic Academy, and after some local stage work landed on the third national tour of “Hamilton.”

Armstrong didn’t lean into his family name to get ahead. Media interviews when he broke out didn’t even mention his famous pedigree. “I never, surprisingly, thought to ever lead with that,” he says. “In this particular show, it’s inescapable.”

Iglehart, also a co- director, recalls Armstrong walking into the rehearsal room and stating his name. “We’re like, ‘How cool.’ He’s like, ‘No, no, I’m related.’ ” He got the job based on his voice and acting — not his name.

“A Wonderful World” has been a way to honor not only Louis Armstrong but also Brandon’s grandfather, Louis Henry Armstrong, who told stories about his proud lineage and would often blast jazz music when picking Brandon up after school.

“I just think in those moments how unbelievably proud he would be of something like this,” he says. “It’s like the best kind of therapy every night.”

Nov. 28 birthdays: Producer Berry Gordy is 95. Singer Randy Newman is 81. Actor Ed Harris is 74. Actor S. Epatha Merkerson is 72. Singer Kristine Arnold is 68. Actor Judd Nelson is 65. Comedian Jon Stewart is 62. Actor Aimee Garcia is 46. Rapper Chamillionaire is 45. Actor Mary Elizabeth Winstead is 40.