Twelve-bar blues is the classic blues form. It is structured in three four-bar phrases that follow a particular chord progression based on four bars of a tonic chord, two bars of a subdominant chord, two bars of a tonic chord, one bar of a dominant chord, one bar of a subdominant chord, and two bars of a tonic chord. There are many variations of this basic progression of harmonies. The deceptively simple structure can support almost limitless complex musical ideas.