Blues
Blues  is based on a simple pattern. “Blues is easy to play, but hard to feel”-Jimi Hendrix. A common form for the blues is a repeating 12 bar form (each bar has 4 beats). This is put together as 3 phrases or sections, each being 4 bars long. The basic blues only needs 3 chords, one based on the first note of the scale - the tonic chord, one based on the fourth note of the scale - the subdominant chord, and one based on the fifth note of the scale - the dominant chord. The first section or phrase of 4 bars only uses the tonic chord. A common  variation is to use the subdominant chord in the second bar. The second phrase has 2 bars (8 beats) of subdominant chord then 2 bars of tonic chord. The third phrase has 1 bar (4 beats) of dominant chord, 1 bar of subdominant chord and finishes with 2 bars of tonic chord.