The 12 bar blues is the most commonly occurring chord progression in the blues. It has been used by blues musicians since the beginning of the 20th century and features in some of the most famous blues songs of all time, including ‘ Sweet Home Chicago ‘, ‘ The Thrill Is Gone ‘ and ‘ Pride and Joy ‘ (amongst countless others).

Two examples of 12-bar blues using that chord sequence are – “Johnny B. Goode” by Chuck Berry and “Crossroads” by Robert Johnson below.

Below is an example of a 12-bar blues stanza from "Dust My Broom," as performed by Elmore James, and broken down by bars (measures), beats, chords, and lyrics: In each 12-bar stanza, the third four-bar segment (in the example above, the 9-12th bars), serves to resolve the previous four-bar segments.