Over The Rainbow is ranked as the number one song in American films - but it nearly failed to make the final cut of its film, The Wizard Of Oz.
The song was designed to showcase Garland's singing talents
BBC World Service's Masterpiece programme profiles one of the most iconic songs ever written.
In 2004, Over The Rainbow was ranked top of a list of the 100 Greatest Songs in American Films by the American Film Institute.
Immortalised by Judy Garland, the 16-year-old actress who performed it 1939's The Wizard Of Oz, it was the last song she sang live 30 years later in Copenhagen, three months before her death from a drugs overdose.
The song was written by lyricist Yip Harburg and composer Harold Arlen - but the duo were a surprise choice after a highly-publicised search by the studio making the film, MGM.
"Everybody was vying for it - that's why everyone was amazed when they got the film," Ernie Harburg, Yip's son, told BBC World Service's Masterpiece programme.
"Everybody was amazed. The Gershwin team, and others like them, were far more entrenched in Hollywood than my father and Harold were."
Somewhere Over The Rainbow was the last of the Wizard Of Oz songs that Harburg and Arlen wrote.
The pair had completed the whole score except for the most important song, which was to set the film off and establish the character of Dorothy.
"They began to write it, but their contract had run out by then - so they were truly working on their own, without getting paid," Harburg recalled.
HARBURG'S PROPOSED TITLES
I Want To Be On The Other Side Of The Rainbow
I Want To Get Across The Rainbow
I Wish I Was On The Other Side Of The Rainbow
"Yip wanted Harold to come up with the music. They were both aware this was a young girl in trouble and wanted to express a yearning to go some place, and get out of trouble."
Ernie Harburg said Arlen was driving with his wife when the first notes of the song came to in.
Immediately, he called to her to stop the car, and then jotted down the first few notes of the long line melody set of bars that open up the song. Soon afterwards, he completed the rest of the song.
However, the song was "almost killed" by Yip Harburg when he first heard it.
"Harold played it in a very symphonic way - he was good at the piano - and Yip was frightened," Harburg explained.
"He said, 'Harold, this is a little girl - this isn't an opera singer'."
At this point, the duo called in their good friend Ira Gershwin, one of the most famous names in Hollywood music composing.
Gershwin's advice was to "take out all those large chords, and try to play it in a simple way, and a popular way."
When he did that, Harburg added, Yip said " yes - that's it."
Yip Harburg had an "enormous capacity" to grasp the basic emotions of the characters he wrote, said John Frick, co-author of a book released for the 50th anniversary of The Wizard Of Oz.
When he wrote Over The Rainbow, Yip Harburg noted that the first chapter of L Frank Baum's original novel - which predated the MGM film by 38 years - used the word "grey" nine times in three pages, to describe Kansas during the drought.
"Everything is grey around Dorothy - and Harburg took that to another level, thinking, if everything is grey, the only thing of colour in Dorothy's life would have been the rainbow," Frick said.
Meanwhile, as well as being written to establish the mood of the film, the song was also required to be a showcase for the talents of Judy Garland.
The song posthumously became associated with Eva Cassidy
"When they sat down to write Over The Rainbow, they were writing for somebody who had the vocal range and the sincerity and the capability of communication that those in the know in Hollywood already felt she had," explained Ken Sefton of the Judy Garland Club.
Ernie Harburg said that, while fitting a song to the singer is a mark of great songwriters, "neither Yip nor Harold, nor anybody, was prepared for the occasion when Judy sang that song on the set for the first time."
However, after all the effort, the song very nearly did not make it to the final cut of the film.
Following rough cut screenings, director Victor Fleming felt Over The Rainbow needed to be taken "because it's too long and it's too difficult."
"He was a big man, and he talked in a big voice, and he wanted that song out," Ernie Harburg said.
"Yip and Harold were out of their minds."
However, the pair prevailed on Louis B Mayer, the head of MGM, to put it in again.
Ernie Harburg said another director then "threw it back out," before Mayer said "'Oh, let the boys have their song.'
"That's the reason why the song was allowed in the picture," he added.
"And of course it became one of the world's great, great songs."