Lead singer Thom Yorke wrote this. He based it on a song by the Jazz player Charles Mingus called "Freedom."
This originally contained handclaps, but the group didn't like how they came out and erased them.
Radiohead performed this at some of their shows before releasing it on the album. It was known as "Egyptian Song."
Their albums Kid A and Amnesiac were recorded at the same time, but Amnesiac was released a few weeks later.
In 2003, this was used in a public service announcement for forest fire prevention in the US. Radiohead never allows their music to be used for commercial purposes, but Thom Yorke thought this was a good cause so he let them use it for $1.
This was written by Thom Yorke after a visit to an exhibition of Egyptian art, during a two-week sojourn in Copenhagen in 1999. He told MTV: "That song literally took five minutes to write, but yet it came from all these mad places. [It's] something I never thought I could actually get across in a song and lyrically. [But I] managed it and that was really, really tough. [Physicist] Stephen Hawking talks about the theory that time is another force. It's [a] fourth dimension and [he talks about] the idea that time is completely cyclical, it's always doing this [spins finger]. It's a factor, like gravity. It's something that I found in Buddhism as well. That's what Pyramid Song' is about, the fact that everything is going in circles."
According to Colin Greenwood, it was the image of "people being ferried across the river of death" that most affected York. This is reflected in the song's many references to Dante's imaginary journey through Hell, Purgatory and Heaven, Divine Comedy. These include the black-eyed angels, a moon full of stars and jumping into the river.
Yorke hammered out this track's chord progression on a baby grand piano that he had bought, in rejection of Radiohead's guitar-led past.
The siren - like sonic undertow was produced by Jonny Greenwood's ondes Martenot, an unusual Theremin-like device invented in 1928.