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Last time (Lesson #3) we were working with the full ii-7 / V7 / IM7 / VI7 turnaround in the key of F (Ex. 1). I mentioned that the VI7 chord is a “secondary dominant”, a chord that acts as a V of a chord other than the I in whatever key we happen to find ourselves. In the key of F the VI7 is D7, which is the V of ii (G-7), often notated as V7/ii-7 (the “five of two”). In practice, however, the dominant chord on VI is so common that we don’t bother with the fancy name – we just say VI7.
The other new piece of harmony I introduced last time was the flatted 9th, probably the most common addition or “alteration” to the dominant chord in jazz. On a D7 the 9th would be E; take that note and lower it a half step and you have Eb, the flatted 9th. The natural 9th E, while adding a nice color to the otherwise drab 7th chord, doesn’t alter the scale from which the chord is derived – D mixolydian (Ex. 3).
But as soon as you alter the 9th by either flatting or (as we shall see in a moment) sharping it, the whole harmonic landscape is altered as well. There is no Eb in D mixolydian, nor is there an E sharp (better known as F natural – the sharp 9th). If the harmony calls for an altered 9th, the natural 9th sounds like a wrong note, and playing it makes you sound like a newbie or like someone who isn’t listening.
Fortunately there is a scale that contains both altered 9ths, and here’s how its derived: Take a look at the D7b9 chord (Ex. 2). If you eliminate the root, the remaining four notes (F# A C Eb) spell a diminished 7th chord – all minor 3rds. The scale that connects those chord tones is called the diminished (or octatonic) scale. (Ex. 4). When you apply that scale to the D7b9 it works beautifully and gives the dominant sound a whole new harmonic flavor.
When you compare the mixolydian to the diminished scale (Ex. 3 & 4), you’ll notice that the natural 9th (E) is replaced by both altered 9ths (Eb and F natural) and the perfect 4th (G) is replaced by the sharp 4th (G#). These “color tones” are powerful notes in the jazz vocabulary; they add delightful tension to the dominant 7th, which makes the resolution to I that much more satisfying.
We’re just scratching the surface of diminished harmony; its a deep well of harmonic and melodic material. There are several books on the subject, as well as many recognizable licks using this sound.
So start listening for the altered 9th – you’ll hear it everywhere. The sharp 9th is also very common – it has a very bluesy flavor, since it contains both the major and minor 3rd (which is the same as the sharp 9th). Ex. 5 gives you a couple of ideas on how to use the diminished scale as part of a bass line. There’s space for you to continue developing your sophisticated bass lines on the remainder of the page. Happy walking!