King Crimson is a band that was founded in 1969 and, although it has undergone many changes, is still around today. This piece is from their Starless and Bible Black album released in January 1974. The band membership was quite unstable at this time but I'm pretty sure this live recording features Robert Fripp (a founding member) on guitar, John Wetton on vocals and bass, Bill Bruford (who had left Yes for King Crimson) on drums, and David Cross on violin and keyboards. The keyboard is a mellotron, which was pretty much the key instrument for any number of progressive bands at the time. This live recording is significantly different than the album version. I'm pretty sure that the violin work in the beginning of the live version was actually Fripp on guitar in the album version. Also there a long, and perhaps improvised, instrumental section in the middle. This is a long and difficult piece but if you like early 1970's progressive music, you've got to love this.
Update: 12/26/13. I see that the original video I posted here has been removed. Here's another.