Marni Nixon passed away this week at age 86. She sang in some of the most popular movies—especially musicals—of the all time. You’ve definitely heard her voice, but you probably don’t know her name or what she looks like. Here’s why.

Born in California in 1930, Nixon was a classically trained soprano, boasting an incredibly clear tone and wide range similar to that of an opera singer. She sang with the New York Philharmonic and was a frequent collaborator of composer Leonard Bernstein, but Nixon primarily worked in film. When movie musicals were still a major film genre in the 1950s and ‘60s, the studios would cast actresses for their acting chops or box office bankability—whether or not they could actually sing. No matter, because the producers would hire Nixon, who would sign a non-disclosure contract agreeing that she’d never tell reporters that it was really her voice behind Hollywood’s leading ladies. But she was so widely used in Hollywood that her identity was hardly a secret. (It also explains why all of the female leads in classic movie musicals all sounded so much alike.)

The King and I (1956)

Deborah Kerr earned an Oscar nomination for her portrayal of Anna Leonowens opposite Yul Brynner as the King of Siam. She did the acting, and Nixon did the singing, including the standard “Getting to Know You.” For her contributions, Nixon was paid standard session singer fees, for a grand total of $420.

West Side Story (1961)

In The King and I, Kerr was aware someone else would do her singing. That wasn’t the case on the set of West Side Story. Natalie Wood (Maria) reportedly had no idea that her singing was going to be replaced in the final film with Nixon’s takes. That’s Nixon singing “I Feel Pretty” among other songs. She also provided Rita Moreno’s parts in the “Tonight” quintet. Nixon asked for soundtrack royalties, but she didn’t get them. However, composer Leonard Bernstein privately signed a contract with Nixon giving her ¼ of 1 percent of his royalties.

My Fair Lady (1964)

In 1964, the film adaptation of My Fair Lady was so popular that it was revived in New York. Marni Nixon played the lead role of Eliza Doolittle. That same year, she provided the singing for the character in that same movie adaptation, as producers decided Audrey Hepburn didn’t have the vocal chops for songs like “I Could Have Danced All Night.”