The Key of C, Blues Bass

C major harmonises to C – Dm – Em – F – G – Am – B°; C natural minor to Cm – D° – Eb – Fm – Gm – Ab – Bb.

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.