The 7♭5♭9 chord is an example of a 7alt chord, where 7alt is defined as a chord with both an altered fifth and an altered ninth. I like to define 7alt chords as any dominant 7th chord with an altered 9th, 11th, 5th OR 13th.
It just so happens that a 7♭5♭9 chord equals a 7#11 on the flatted 5th. As an example, the notes in G7♭5♭9 are the same as in a D♭7#11. I cover the 7♭5♭9 and 7#11 chords for the keys of G and D♭.
The 7♭5♭9 & 7#11 chords in detail
The 7♭5♭9 chord is one of the 7alt chords used in jazz. 7alt chords are defined as a dominant 7th chord (major 3rd & flat 7) that have an altered 5th AND an altered 9th. Which means there are only 4 of them:
Check out my article on the G7#5#9 chord and the handful of closed and open chord shapes for a 7#5#9 chord.
I like to make up my own definitions, so I define altered 7th’s as any chord with a major 3rd and a flat 7th (dom7) that has an altered 9th, 11th, 5th, OR 13th. The available altered chord tones would then be ♭9, #9, #11, ♭5, #5 or ♭13.
That definition adds a lot more chords to the “altered” 7th category. You can also look at it as any dominant 7th chord that can not be built from the major scale. There are only 4 dominant chords that can be built from a major scale and none of them have altered 5ths or extensions: 7, 9, 11 & 13.
I bring up my definition of altered 7ths because the 7♭5♭9 chord is equal to a 7#11 on the flat 5. Here are the chords in detail:
Chord tones: G-B-D♭-F-A♭
Chord intervals: R-M3-d5-m7-m2 = 1-3-♭5-♭7-♭9
Alternate names: 7alt, 7 (♭5, ♭9), 7♭5(♭9), 7 flat 5 flat 9, 7 ♭5 ♭9, 7(♭5,♭9),
Scales that build a 7♭5♭9: 7th-degree melodic minor (Altered scale), and the tonic of the HW diminished scale
Resolve tendency: m2, M2, P4, P5, m7, and M7. Or for G7♭5♭9, that would be A♭ the ♭9, A the 9, C the 4th, D the 5th, F the ♭7, and F# the major 7th. You also look at the resolve tendency as down or up a semitone AND whole tone from the root, as well as to the IV & V. This chord has a lot of tendencies. The tritones point to D, E, A♭, & B♭ and they have the strongest resolution.
Chord tones: D♭-F-A♭-C♭-G
Chord intervals: R-M3-P5-m7-A4 = 1-3-5-♭7-#11
Alternate names: 7+11, 7(#11), 7(+11) ,7 sharp 11, dom7(#11)
Scales that build a 7#11: 4th-degree melodic minor (Lydian Dominant), and the tonic of the HW diminished scale
Resolve tendency: Same as the 7♭5♭9 chord
The Half-step Whole-step diminished scale would be a great choice for either chord and of course the appropriate melodic minor modes.
The 7#11 versus the 7♭5 chord
A lot of people call a 7♭5 chord a 7#11. With that logic, a 7#5 is a 7♭13. Yes, they sound and perform similarly but they are not the same chord. Here is the main point: since a 7#11 equals a 7♭5♭9 on the #11, you can’t call a 7#11 without a 5th (aka, 7♭5) a 7♭5♭9. Understand?
Why would you call a 7♭5 a 7#11?
The 1st note that gets dropped in chords is the perfect 5th, but you would never drop an altered 5th. Can you imagine someone calling a chord in their song a 7#5 no fifth? You can play a dom7 without the 5th, but you assume that it’s the perfect 5th that was dropped.
That same logic dictates that you can’t, or should not, drop the perfect 5th in a chord that has either a #11 or a ♭13. The reason should be obvious.
Just call a 7♭5 a 7♭5, not a 7#11.
Closed & Open 7♭5♭9 guitar chord shapes for G & D♭
I have 3 closed chord shapes, 2 open for G and 4 open for D♭. I do have 3 voicings for a 7♭5♭9 without the 3rd which is equal to an add#11 chord, but I’ll leave them for another article maybe.
Here is a chord diagram of the symbols I use in my chord blocks:
Notes on the 7♭5♭9 chord voicings:
CLOSED: I like #1 & #3 the best. #2 shows barring all 6 strings even though the 6th string is not played – it’s easier to hold that way.
OPEN: #2 G7♭5♭9 is hard and really dissonant – you can drop the B on the low E. And since the bass note is not the root, this is also a voicing for a D♭7#11. I prefer all the 7♭5♭9 except #1 for G and #3 for D♭.
Closed & open 7#11 guitar chord shapes for G & D♭
I have 3 closed chord shapes, 3 open for G and 1 open for D♭.
Notes on the 7#11 chord voicings:
CLOSED: #3 is the same voicing as #3 closed for 7♭5♭9. I like #1 & 3 but #2 is difficult to hold.
OPEN: I like all the open voicings except #3 for G.
If you ask me, these are some “nasty” chords, but they sound good once you get used to their dissonance. Jazz players can make any chord sound good for various reasons I won’t get into.
This site is about every guitar chord with the point of using the chords in original songs. If you play jazz, then hopefully I showed you some new voicings.
If you are a singer-songwriter, then you can try the 7♭5♭9 or 7#11 in blues tunes. They sound good for the I, IV or V chords, but the tonic chord (I chord) makes the most sense given the flat 5 in a blues scale.
But take another look at the resolution tendency for the chords and then play around with substituting them. Or use either chord as a substitute for the V7 chord in your song. They may just give you a nasty sound that makes the return to the I chord more interesting.