Why were John Coltrane's Giant Steps changes so hard to play?

Giant Steps is hard because its chords do not move the way jazz chords normally move. Most jazz harmony runs around the circle of fifths, as in the ii–V–I progression. Coltrane's piece instead sets key centers a major third apart, root movement by thirds, which is unusual in jazz and which creates an augmented triad. Musicians who spent years learning to follow the circle of fifths suddenly face a pattern with no familiar signpost.

There is a second difficulty: the three keys are equally spaced. Because the twelve notes are equidistant, three tonic notes a major third apart, such as C, A flat and E, produce a destabilizing effect, and the identity of the tonal center can only be determined by the closure of the composition. Drawn on the circle of fifths the three keys form an equilateral triangle, and there are only four unique thirds cycles.

The album Giant Steps came out on Atlantic Records in February 1960, and jazz musicians have used its title track as a practice piece ever since: and its changes are considered a kind of ultimate harmonic challenge. Coltrane did not conjure the idea from nowhere. Third-related harmony had existed since the Romantic era, and the bridge of the 1937 standard Have You Met Miss Jones? already modulates by major thirds. He also studied Slonimsky's Thesaurus of Scales and Melodic Patterns. What Coltrane did was make the cycle the backbone of a whole tune, and his improvisation on it exemplifies the melodic phrasing that came to be known as sheets of sound.