"Reckoner" is a song that explores themes of separation and connection, with its lyrics suggesting a central idea of things leading to and then moving away from a single point. The singer described it as a love song, albeit "sort of," and considered it the heart of the album it appears on, In Rainbows. The song features a distinctive falsetto vocal, "frosty, clanging" percussion, and a meandering guitar line. At its heart, it plays like a meditative reflection on relationships and the forces that bind and unbind us.