The classical (standard) blues form consists of 12 bars (measures). This 12-bar structure is one of the most recognisable and widely used chord progressions in Western popular music. It is divided into three 4-bar phrases, typically following this harmonic pattern:
Bars 1-4: Tonic chord (I)
Bars 5-8: Subdominant chord (IV) for two bars, then back to tonic (I) for two bars
Bars 9-12: Dominant chord (V) for one bar, subdominant (IV) for one bar, then tonic (I) for two bars (often with a turnaround)
This 12-bar blues progression has been the foundation of blues music since its earliest forms in the late 19th and early 20th centuries and has influenced jazz, rock and roll, R&B, and many other genres.
An 8-bar form exists in some blues variants but is not the classical standard. 16-bar and 32-bar forms are associated with other popular song structures (32 bars, for example, is typical of the AABA Tin Pan Alley song form), not the standard blues.
