Twelve-bar is so named because a chorus of the music is typically twelve bars long. (In fact, there is not a hard and fast rule – the term ‘twelve-bar music’ also has melodic, harmonic and rhythmical implications independent of chorus length. Early blues music in particular was less bound by the nominal twelve-bar length.)
The structure of twelve-bar music is expressed in the disposition of its three essential chords over the twelve-bar chorus. This structure can be more or less complicated. The following examples offer a sample less-to-more-complicated ordering, but you are sure to find variations which do not fit neatly into the sequence.
Examples One to Three are shown in G, but the Roman numeral system numbers I, IV, V, rather than the chord symbols are the ones that will really give you a creative grasp.
(Note that simpler chord sequences than this are certainly possible.)
Either of the developments shown here (bar 2 and bar 12) could appear on their own.
Here we start to see the development of the ‘turn’ – bars 11 and 12.
This chord sequence shows the introduction of the circle of fifths.
The circle of fifths (and the jazz blues) is a study in itself and beyond the scope of this volume. If you are interested in pursuing a study of harmony, refer to the Musicarta Key Chords Vol.1 study package and stay posted for news of the forthcoming Volume 2 – which will venture into the circle of fifths territory.
These diagrams are of course only representations of what you hear – and of skilful listening, in terms of what you expect to hear and will quickly be able to identify.
You would hope to be able to look at these diagrams, start up a twelve-bat texture in you ‘mind’s ear’ and hear a twelve-bar with that particular chord structure in your head – and play one at the keyboard.