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Be they in major key or minor key, such I-IV-V chord-progressions are extended over twelve bars in popular music—especially in jazz, blues, and rock music. For example, a "twelve-bar blues" progression of chords in the key of E has three sets of four bars:
this progression is simplified by playing the sevenths as major chords. The twelve-bar blues structure is used by McCartney's "3 Legs", which was noted earlier.
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