What song does Audrey Hepburn strum on her guitar in "Breakfast at Tiffany's"?

Audrey Hepburn, as stylish social Holly Golightly, sits on her New York City fire escape, playing the wistful Henry Mancini tune "Moon River" in “Breakfast at Tiffany’s” (1961). According to the BBC, the film’s production studio, Paramount, originally thought they would need to dub Hepburn’s real voice in the film since she was not a trained singer. But Mancini penned “Moon River” to sit right in the sweet spot of Hepburn’s range.

“Henry was at home one night and we were watching ‘Funny Face’ in which Audrey Hepburn sings ‘How Long Has This Been Going On?'” Mancini’s widow, Ginny, recalled. “He went to the piano and started playing it, and knew she could sing something in that range.”

It took about a month and a half before Mancini put down the first three notes, and then in a half-hour, he had written the "Moon River" melody. Mancini asked Johnny Mercer to write the lyrics, and they were ultimately awarded the 1961 Oscar for Best Original Song.