Using the 12-bar blues form to improvise
Nobody knows exactly how the musicians around the turn of the century came to settle on a format for the blues, but at least by shortly after 1900Â it was obvious that the blues were generally performed in a 12 measure format which came to be referred to as simply “the blues.” It’s possible to have “blue notes” (flat 3rds, 5ths, 7ths) in any popular song, but when jazz musicians speak of the blues, they usually mean the 12-bar format.
Here is the structure of the 12-bar blues:
*4 bars of the I chord (the root chord of whatever key you’re playing in)
*2 bars of the IV chord (often with the 7th)
*2 bars of the I chord
*2 bars of the V chord (almost always with the 7th) or 1 bar of the V chord followed by 1 bar of the IV chord.
*2 bars of the I chord
Then repeat for as many times as desired.
Over this chord structure the musicians improvised a melody based on the given chords and scales of those chords. He or she is free to play whatever they want to play, consistent with his ability and taste. Typically the blues is performed by a small group, often made up of a piano, bass, drums, and 1 or 3 solo instruments such as a sax or trumpet.