Although the lyrics to "Purple Haze", which opened the US edition of "Are You Experienced", are often misinterpreted as describing an acid trip, Hendrix explained: " was all about a dream I had that I was walking under the sea." He speculated that the dream may have been inspired by a science fiction story about a purple death ray. Redding stated that Hendrix had not yet taken LSD at the time of the song's writing, which was after a gig in London on December 26, 1966. The first draft of the lyrics was exceedingly long, so Chandler and Hendrix reduced its length to something appropriate for mainstream pop music. It opens with a guitar/bass harmony in the interval of a tritone that was known as the "diabolus in musica" during the time of the Spanish Inquisition. The Catholic Church prohibited medieval composers of religious music from using the tritone, or flattened fifth, because as musicologist Dave Whitehill wrote: "to play it was like ringing Satan's doorbell." In the opinion of the author Ritchie Unterberger, the opening riff has "become a permanent part of rock's vocabulary." Whereas "Rolling Stone" described the song as the beginning of late-1960s psychedelia, the authors Harry Shapiro and Caesar Glebbeek identified Hendrix's use of R&B, funk, and soul elements in the track.