Matt Bomer portrays David Oppenheim in 'Maestro.' Here's what to know about Leonard Bernstein's close friend.

• In the film "Maestro," Leonard Bernstein is shown in a romantic relationship with David Oppenheim.
• The two knew each other in real life, and were lifelong friends.

Leonard Bernstein's career and personal relationships are the focus of the new Netflix film "Maestro," starring Bradley Cooper and Carey Mulligan. While the majority of the film follows Bernstein's (Cooper) complicated marriage to Felicia Montealegre (Mulligan), viewers also get a glimpse of his friendship with clarinet player David Oppenheim (Matt Bomer).

"Maestro" portrays Bernstein and Oppenheim as close friends (the two were both involved in the New York classical music scene in the 1940s and 1950s) and also lovers. Their relationship grows strained after Bernstein begins pursuing a relationship with Felicia.

But what was the relationship between Bernstein and Oppenheim like in real life? Here's what we know about the lifelong friendship between the two musicians.

Bernstein and Oppenheim met when they were young musicians in New York

While the exact date of Bernstein and Oppenheim's first meeting hasn't been widely reported, the two appear to have crossed paths when they were both living in New York City in the early 1940s.

The Library of Congress has a wide-ranging collection of Bernstein's correspondence with friends and family, and the first letter they have from Oppenheim to Bernstein is dated December 23, 1942. Addressed to "Lenny Bernstein," the letter simply says: "See you Thursday afternoon or evening. Dave."

Bernstein and Oppenheim's friendship continued in the years after World War II, when Oppenheim was the first clarinetist for the New York Symphony Orchestra, and Bernstein was regularly conducting concerts at the Philharmonic. As Oppenheim's obituary in the New York Times noted, he studied at the Tanglewood Music Center for a few years after the war as well, working with composers including Bernstein. Bernstein regularly taught and conducted music at the Massachusetts music center for most of his life.

After a stint at Columbia Records for most of the 1950s, Oppenheim made the jump to television, writing, directing and producing music-focused shows for CBS. He later became the dean of New York University's School of the Arts, which, under his direction, overhauled its curriculum and secured donations from board members to become what is now known as the Tisch School of the Arts.

Oppenheim was married three times, and died in 2007. He had a son with his first wife, Judy Holliday, and two children with his second wife, Ellen Adler. He and his third wife, Patricia Jaffe, married in 1987.

The letters between Bernstein and Oppenheim in the Library of Congress' collection span nearly 50 years

Bernstein died in 1990, and the last letters from Oppenheim to his lifelong friend in the Library of Congress' collection are formal thank you notes to Bernstein for donating scholarship money to NYU Tisch.

The sexual nature of Bernstein and Oppenheim's relationship hasn't been widely reported on, and while "Maestro" depicts the two as on-again, off-again lovers during their younger days, it's unclear if or when their relationship turned sexual in real life.

However, a letter from Oppenheim to Bernstein, dated February 7, 1946, seems to suggest that the relationship between the two men grew to become emotionally rich and genuinely fulfilling, regardless of whether or not they were lovers.

"Here, after years of confidences and intimacies, I still feel so very far from knowing you the way I would like to," Oppenheim wrote to his friend. "New York promises great things though, and I am confident. Let's have a grand relationship, shall we?"

"Maestro" is now streaming on Netflix. Watch the trailer below.