The lyrics of "21st Century Schizoid Man" were written by Peter Sinfield and consist chiefly of disconnected phrases which present a series of images. All three verses follow a set pattern in presenting these images. The first line of each verse presents two relatively vague images (e.g. "iron claw", "death seed"). The second line is a single image, often more specific than the first two, and the third line approaches an actual sentence. The fourth and last line of each verse is the song's title.

The song makes reference to the Vietnam War with the lyrics "Politicians' funeral pyre/Innocence raped with napalm fire". Before a live performance of the song on 14 December 1969, heard on the live album Epitaph, Robert Fripp remarked that the song was dedicated to "an American political personality whom we all know and love dearly. His name is Spiro Agnew."

Clocking at nearly seven and a half minutes, the song is notable for its heavily distorted vocals sung by Greg Lake, and its instrumental middle section, called "Mirrors". Most of the song is in either 4/4 or 6/8 time, save for the end of the song, which is in free time. Fripp explained his guitar solo to Guitar Player magazine in 1974: "It's all picked down-up. The basis of the picking technique is to strike down on the on-beat and up on the off-beat. Then one must learn to reverse that. I'll generally use a downstroke on the down-beat except where I wish to accent a phrase in a particular way or create a certain kind of tension by confusing accents, in which case I might begin a run on the upstroke." The atonal solo was rated number 82 in Guitar World's list of the Top 100 Greatest Guitar Solos in 2008. British Prime Minister Tony Blair praised the song and especially the guitar solo. The song encompasses the heavy metal,jazz-rock and progressive rock genres.

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