The song begins quietly, with just Paul McCartney's voice and a piano. After the first verse, the song builds - guitars, back-up vocals, Ringo Starr's drums.

Then there's the extended outro: A chorus chanting "Na, na, na / Hey Jude" something like 18 times accompanied by the 36-piece orchestra. The song is more than seven minutes long but doesn't feel like it.

"Hey Jude," even for the first-time listener, is unforgettable. This rock 'n' roll gem, its composer McCartney says, was inspired by the divorce of John and Cynthia Lennon, who died Wednesday at 75.

"Jude," it turns out, is Julian Lennon - the couple's son, who was about 5 when his parents divorced in 1968. McCartney came up with idea while driving out to see Cynthia Lennon, who was expelled from the Beatles' inner circle after John Lennon met Yoko Ono, his future wife.

"I started with the idea 'Hey Jules' - which was Julian - 'don't make it bad, take a sad song and make it better,' " McCartney said, as Howard Sounes wrote in Fab: An Intimate Life of Paul McCartney in 2011. "Hey, try and deal with this terrible thing. I knew it was not going to be easy for him. I always feel sorry for kids in divorces . . . I had the idea [for the song] by the time I got there. I changed it to 'Jude' because I thought that sounded a bit better."

McCartney said he intended to take out the song's most mysterious line - "the movement you need is on your shoulder" - but John Lennon wouldn't have it.

"I was playing the song to John, and I said . . . 'I'll be taking that out.' He said, 'What for? . . . that's the best line in it, man,' " McCartney recalled.

John Lennon and Cynthia Powell met as teenagers in Liverpool in the 1950s, but they were an odd pair - she reserved, he an extrovert. And the troubled musician and art student, who Cynthia Lennon claimed once hit her when he saw her dancing with original Beatles bassist Stu Sutcliffe, wasn't exactly a catch.

"[John] was a very jealous young man at the time, and he had a lot of pain inside," she said in 2005.

The couple married when she became pregnant with Julian in 1962. John Lennon, who played a show on his wedding night, wasn't all that enthused.

Then the marriage a Beatle didn't want became the marriage the Beatles didn't want. A teen idol with a wife and son wouldn't go over well with fans, and what Cynthia Lennon later deemed her "undercover existence" was kept hush-hush.

"I have read so many books and seen so many films, and it's like we don't really exist," she told Good Morning America in 2005. "We are like walk-on parts in his life."

Although John Lennon wrote "Lucy in the Sky with Diamonds" about a picture Julian drew, he wasn't exactly a doting father.

"I've never really wanted to know the truth about how Dad was with me," Julian Lennon said. "There was some very negative stuff talked about me . . . like when he said I'd come out of a whiskey bottle on a Saturday night. Stuff like that. You think, 'Where's the love in that?'

"Paul and I used to hang about quite a bit . . . more than Dad and I did. We had a great friendship going, and there seems to be far more pictures of me and Paul playing together at that age than there are pictures of me and my dad."

Once Ono was on the scene, the marriage couldn't last. When Cynthia Lennon returned from a vacation and caught her husband with the avant-garde artist at the family's home, it was the end.

"He said, 'Hello,' " she said. "I didn't get one word out of Yoko."

But the fraught relationship resulted in a song that John Lennon called a "masterpiece." Of course, he didn't think the song was so much about his son as about himself, with McCartney encouraging him to leave their partnership.

"I always heard it as a song to me," Lennon said in 1980, not long before his death. "Yoko's just come into the picture. . . . [Paul is] saying, 'Hey, Jude - hey, John.' Subconsciously he was saying, 'Go ahead, leave me.' "