The Key of Ab, Blues Bass

Ab major harmonises to Ab – Bbm – Cm – Db – Eb – Fm – G°; G# natural minor to G#m – A#° – B – C#m – D#m – E – F#.

A blues takes those tables as a starting point rather than a rule: it plays the tonic, the subdominant and the dominant all as dominant seventh chords, which no major key contains on its own — the first place the idiom and the textbook part company.

The bass carries a repeating figure under every bar that moves with each change of chord and tells the rest of the band the change has arrived. It seldom sounds the whole chord: it picks which tones to step through and in what order, and the band hears the harmony from whatever note it lands on at the top of the bar.