The Half-Whole Truth: How to Solo Over a 7#9 Chord with the Half-Whole Diminished Scale

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PHOTO: Cindy Moorhead

Ever find yourself improvising over a static (unchanging) dominant-seven sharp-nine (7#9) chord? In this scenario, most guitarists will instinctively draw from the trusty five-note minor pentatonic scale (intervallically spelled 1 b3 4 5 b7) or the six-note blues scale (1 b3 4 b5 5 b7), both of which sound solid, satisfying, and cool when played over this chord. But you may reach a point where you find yourself looking for other options beyond these familiar sounds, in order to change things up and add harmonic color and a splash of haunting, jazzy dissonance to your lines. In this lesson, I’ll show you an effective way of doing that using an eight-note entity called the half-whole diminished scale.

As its name implies, the half-whole diminished scale is built in alternating half steps (equivalent to one fret) and whole steps (equal to two frets), it’s intervallic spelling being 1 b2 b3 3 b5 5 6 b7. Many musicians find it helpful to think of the b2 and b3 as the b9 and #9, respectively. Starting on an A root note, the scale would be spelled A Bb C C# Eb E F# G.

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Fig. 1a illustrates a standard go-to guitar voicing for A7#9 (A C# E G B#), for which the fifth of the chord, E, is not included, as it is both impractical to finger and harmonically unnecessary to define the chord’s signature quality. As mentioned earlier, when soloing over this chord, most players will instinctively gravitate toward playing lines based on the A blues scale. It’s also common, though less so, for more harmonically conscientious players to add the major third, C#, to this set of notes. Fig. 1b shows the standard fifth-position, two-octave A blues scale “box” pattern, with the C# note added in both octaves. Notice that this set of notes includes all the chord tones of A7#9, so it makes sense why this scale choice works well and sounds pleasing over this chord. But chances are you already do this, so lets move on.

Take a look at Fig. 1c, which depicts the A half-whole diminished scale played in this same position. Notice that this scale pattern is very similar to our previous one, the differences here being that there is now no D natural (the perfect fourth of A) and we’ve added the notes Bb (the b2, or b9) and F# (the 6, or 13), the latter of which may be thought of as being borrowed from the A Mixolydian mode (A B C# D E F# G). So one could think of this scale as a kind of “hybrid” blues scale that adds some extra tension tones.

Okay, let’s look at some examples of ways to apply the half-whole diminished scale over a 7#9 chord in various common keys. Ex. 1 is in the key of C and evokes a Robben Ford-type sound. Here we’re playing over a C7#9 chord (C E G Bb D#) and using notes from the C half-whole diminished scale (C Db D# E F# G A Bb), which, by the way, happens to be comprised of the same eight notes as the A half-whole diminished scale we looked at earlier, the only differences being the orientation of the notes around a C root instead of an A root and the Eb note (the b5, relative to A) now being reckoned as D#, the theoretical #9, relative to C.

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Ex. 2 is played over a G7#9 chord (G B D F A#) and uses notes from the G half-whole diminished scale (G Ab Bb B C# D E F). This line incorporates the use of minor second (half-step) intervals contrasted by larger intervals, which creates an interesting overall melodic contour.

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As you’ve probably figured out, the half-whole diminished scale’s structure is symmetrical, as the scale degrees alternate between half steps with whole steps. The scale may also be conceived as being the sum of two adjacent diminished seven arpeggios, rooted a half step apart, which may be thought of as being “upper and lower neighbors” of each other. Going back to the key of A, those arpeggios would be Adim7 (A C Eb Gb) and Bbdim7 (Bb Db E G). (The latter may also be viewed as a rootless A7b9 arpeggio: A C# E G Bb.) This symmetrical construction lends itself to some interesting melodic ideas that are not possible with the blues scale. Ex. 3 is from the David Baker book How to Play Bebop and is considered a “public domain” diminished idea and a stock pattern that moves symmetrically. Patterns based on symmetrical scales are often built from short note groupings, or “cells,” that repeat at different intervals and are chained together to create longer ideas. The idea in Ex. 3 is built from a four-note cell that descends in minor thirds, resulting in a sound that is more mathematical than bluesy, which can be cool in certain situations.

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Ex. 4 is based on a three-note cell that descends in minor thirds. If we think of a set of “target notes” based on a descending diminished seven arpeggio in the key of A, we get A, Gb, Eb, and C. Now play each three-note cell beginning with a note a half step above the target note, followed by the target note then a note a minor third above. For example, if A is the target note, the cell would be Bb-A-C. Now we can do the same thing using the target note Gb. Since the scale is symmetrical, the cell can move up or down in minor thirds. An easy way to do this on the guitar is to move it up or down three frets on the same string group.

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Ex. 5 takes the interval of a major second and moves it around the scale, creating a “space-y” kind of sound. As you can see from this and the previous example, once you have a cell, you can begin experimenting by moving it around within the scale, which creates an appealing sense of continuity.

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As is the case with many scales, we can build triads from the notes of the half-whole diminished scale by stacking up them in thirds. For example, the A diminished scale may generate the major triads A (A C# E), C (C E G), Eb (Eb G Bb) and F# (F# A# C#), as well as their minor counterparts! Ex.6 uses many of these triads , both minor and major, with a descending pattern as the basis of its melody. Because the pattern here is quite long, you may want to start it somewhere in the middle or use only a fragment of it to play around with.

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While exploring these ideas, try weaving them in and out of the more “inside” minor pentatonic blues-scale approach we discussed at the beginning of the lesson, and mix them in with your own bluesy phrases. This approach can be used to create a cool “inside-outside” kind of sound and add a new dimension to the experience of jamming over a 7#9 chord. They key, of course, is to not just play rote scales, arpeggios, and interval patterns, but to come up with original melodic shapes and interesting note contours that appeal to you. I highly recommend keeping a notebook handy and transcribing any patterns and phrases you like. Doing so can help you organize your thoughts and creative ideas, prevent you from forgetting them, and make it easier to connect old ones with new ones.

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