The 12-bar blues or blues changes are one of the most popular chord progressions in popular music. The blues progression has a distinctive form in lyrics and phrase and chord structure and duration. It is, at its most basic, based on the I-IV-V chords of a key. A 24-bar blues follows the same changes but each chord lasts for twice as many measures. The blues can be played in any key. Mastery of the blues and rhythm changes are “critical elements for building a jazz repertoire” (Thomas 2002, p.