Welcome to the first and most important lesson in 12 Bar Blues Form & Harmony!
Within each 12-bar chorus, there are three 4-bar sub-phrases:
The first 4-bar phrase presents the main rhythmic/melodic theme and establishes a clear sense of tonic harmony I:
Listen to a typical phrase #1:
As you listen, sing the root note of the I chord (Do) out loud.
The second 4-bar phrase is either a repetition or variation or complement of the main theme played over a temporary IV harmony, with a return back to the tonic I:
Listen to a typical phrase #2:
As you listen, sing the root notes of the IV chord (Fa) and the I chord (Do) out loud.
The third 4-bar phrase is almost always different from the first two phrases, but something that complements or answers the first two phrases and leads the listener back to the top of the next chorus. It is played over a temporary V harmony with a return to the tonic I:
Listen to a typical phrase #3:
As you listen, sing the root notes of the V chord (So) and the I chord (Do) out loud.
Now let’s put all three phrases together:
Such phrasing is absolutely essential to playing boogie woogie (and related styles like blues) in authentic style. In order to sound/feel “right”, each 12-bar chorus should be rhythmically and melodically coherent, say something that moves the listener, and lead back to the top of the next chorus or to a logical ending. Think of these three phrases as interrelated musical sentences that tell a story. With some tasty licks in your bag and a commitment to this basic character, your boogie playing will sound right in style.