This song, formerly known as "Feeling Pulled Apart By Horses," was debuted by the band at the Gorge Amphitheatre in George, Washington, on June 23, 2001.
At the golden ratio of In Rainbows, which occurs 2 minutes 49 seconds into "Reckoner," the background vocals can be heard singing "In Rainbows." It was the ancient Greek mathematician Euclid, who first defined the golden ratio, an aesthetically pleasing proportioning within a piece. A mathematical constant, the golden ratio is calculated as approximately 1.6180339887. In a work of art it appears at 1/1.618, or about 61.8% of a way through. On In Rainbows 2 minutes 49 seconds into "Reckoner" is 61.8% of the way through the album.
"Reckoner" was an early working title for the album before it was titled In Rainbows. The album does take its title from this song's lyric: "Because we separate like ripples on a blank shore (in rainbows.)"
Guitarist Jonny Greenwood was asked by Mojo magazine January 2008 if he was aware that In Rainbows was a remarkable record. He replied: "Well, we spent more time on it than we should have done and you go a bit mental after a while. You listen to something like 'Reckoner' and think, maybe this is just a bad break beat. Now I can hear what's good about it. People around us were telling us it was really good, but what do they know? What do we know?" He added later in the interview: "It was when we recorded 'Reckoner' that we felt we'd first got something special on tape."