It's no secret that this was written after George Harrison, who was a huge fan, told John Bonham that the problem with Led Zeppelin is that they don't do any ballads.
Bonzo brought in the idea, and the rest of the band worked with him to put it together. It is was one of the few Zeppelin songs where all four members shared the composer credit.
Jimmy included the first two notes (not chords as it's widely known) of the George's written Beatles song 'Something' in the intro to 'The Rain Song.
Led Zeppelin performing The Rain Song live at
Knebworth Festival, Stevenage, UK, in 1979