The Key of D, Blues Bass

D major harmonises to D – Em – F#m – G – A – Bm – C#°; D natural minor to Dm – E° – F – Gm – Am – Bb – C.

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.