In addition to this chromaticism, blues music also is known for having loose boundaries between major and the parallel minor. Chords are often played with a minor third that slides or bends into a major third. This technique adds to the blues flavor of the sonic texture.
The 12 bar blues form is used primarily in the blues style of music, but is also found in many pop, rock and gospel songs. These styles can tend to be heavily influenced by the blues style. The form is popular among musicians, because the simple repeating harmonic pattern causes it to be easy to memorize. Musicians can play most 12 bar blues songs without a lead sheet. All they need to know is the key of the song.
The basic pattern for the 12 bar blues can be broken into three groups of 4 measures each. The first four measures consist of the I chord. Then there are two measures of IV and two measures of I. Finally there are two measures of V and two measures of I.
In any version of the 12 bar blues (basic or variation) a V chord can be substituted for the final I chord. This substitution is used to lead back to the top when repeating the form. When ending the form, the musicians play a I chord for the final measure.
Another variation is the minor 12 bar blues. In this variation every I and IV chord become a minor i and iv chord. In addition, the last group of four measures consists of a bVI for one measure, a V7 for one measure and then a i for two measures.
This Learning Music With Ray video discusses the 12 bar blues form. In it, I discuss the structure and musical elements typical of this form. I then play through the basic 12 bar blues (and some basic variations) with simple block chords in the key of C major. Finally, I demonstrate several piano playing techniques that help to enhance your performance of a 12 bar blues beyond basic block chords.