The Key of Db, Blues Bass

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

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.