The 12 bar blues originated in the Deep South of the United States at the end of the 19th century, from spirituals, work songs, and chants, the unaccompanied vocal music and oral traditions of slaves imported from West Africa. The name “blues” refers to the “blue devils”, meaning a depressed mood, melancholy and sadness. Also in the titles of many blues-songs we see this reflected: Go Back to Your No Good Man Blues, Blues and Booze, Prohibition Blues, Mean Old Bed Bug Blues etc…
The standard 12 Bar Blues form is the most common structure of blues music. It’s a cliche, but a very effective one (as much as the melodic emphasis on the blue notes is).
The 12 bars are divided into three four-bar frases, in which only the primary chords I7, IV7 and V7 are being used.
Example in C major
Notice the use of dominant seventh chords I7, IV7 and V7. This became very popular during the dixieland and swing eras (1917-1947), and is now considered one of the basic features of the blues-sound (in comparison with the use of Imaj7, and IVmaj7 in jazz). As we already mentioned before the extension of a triad with a minor seventh is quite an old invention. In the blues it seems that the chords on all degrees are extended with a minor seventh. Another important feature is the plagal chord progression V7 – IV7 – I7 in the last four bars, which as we saw is a common progression in folk music, but was not very popular in classical and romantic music (such as melodies for wedding celebrations, Valentines day, wedding anniversaries, birthdays etc…).
The 12-bar blues scheme can be considered the historical blue-print of blues music. (Jazz music also has such a blue-print. It is the song I got rythm by George Gershwin.)