This is a question that’s haunted me in a low-level way since I was in the eighth grade and my guitar teacher in Silver Spring MD first taught me that I could solo over the 12-bar blues changes using the parallel minor pentatonic scale. He probably didn’t use the words “parallel minor” at the time, and I only learned these years later in freshman theory/aural skills. But yes, he was right: not just the relative minor works, but the parallel minor. I knew a little about relative minor, even then. I knew that if I was soloing over a blues in C major, I could safely use the C major scale, and also C major’s relative minor, A minor, and therefore also the A minor pentatonic. But a C minor pentatonic?? Not only didn’t the parallel keys end up clashing horribly as might be expected, but the C minor pentatonic was probably the most “bluesy” sounding choice! How could this be?
While trading solos over a C blues with some cello students today, with a Music Compass on the stand demonstrating the C minor pentatonic, I finally grasped one neat reason why.
Let’s start with the chords in the C major 12-bar blues progression. We need a I, a IV and a V chord, and in the blues all three of these are typically dominant seventh chords. So, in C major, that means a C7, an F7 and a G7 chord. Since I was playing the chord accompaniment on cello, I wanted to reduce each of these four-note chords to the essential notes so my hands didn’t get tired too quickly. I reduced each chord to a dyad of just two notes — the root and 7th — to make sure I was both anchoring the root motion and supplying that dominant 7th sound so characteristic in the blues. So here’s what I was playing for each chord:
If you take all three of these dyads consisting of root notes + sevenths, and combine the notes into one shape, you get… the C minor pentatonic!
This is a big reason why the C minor pentatonic can “float” so nicely over the three dominant seventh chords of the 12-bar C major blues.