Roger Keith “Syd” Barrett co-founded and named the groundbreaking British rock band Pink Floyd. He was dismissed from the group after the release of its second album, in 1968. The late Mr. Barrett (1/6/46 – 7/7/06) was admitted to a hospital with what was rumored—and later confirmed to be—mental illness. David Gilmour took his place as lead singer and guitarist shortly before Mr. Barrett’s breakdown. The song is a tribute to him; the initial letters of Shine, You, and Diamond spell out Syd.

Originally conceived as one entire side of the band’s 1975 concept album, “Wish You Were Here”, “Shine on You Crazy Diamond” had to be re-recorded due to technical problems. At that point, the band decided to record “Diamond” in nine parts, using the first five to begin the album. Thereafter, three other songs appeared, and the final four parts of “Diamond” ended the album. The album’s concept was an exploration of the band’s then-current situation.

Without having communicated with his former Pink Floyd mates for years, Mr. Barrett visited the Abby Road studio on the day the band was recording the vocals for “Diamond”. No one recognized him, as he had gained weight and shaved his head and eyebrows. He simply sat in the studio and did things that the band members considered strange.

Mr. Barrett did not interact with the band members until one of them recognized him after around 45 minutes. When asked about what he had heard, Mr. Barrett said it sounded somewhat dated to him.

More Info: en.wikipedia.org