Giant Steps

Twenty-six chords, six tonal centres, and a reputation as the hardest tune in jazz. The reputation comes from speed: the centres change every two bars, at the fastest tempo in the repertoire.

What almost nobody mentions is that it uses only nine different scales — fewer than many bossas. Giant Steps isn't hard because it's complex. It's hard because there's no time to think, which means the choice has to be settled beforehand.