Use of 7flat9 or Diminished

Question

For years as a guitarist, I used to play diminished chords in place of 7♭9 chords.They sounded very similar, and if the bass (or another player) in the band played the root note, that made it alright in the end.

For example: Foggy Day, and Have you Met Miss Jones, both I've seen written going F to D7♭9 and F to Eb°. Even F to D7! And on occasions, I'll play what I fancy out of them - they all fit.

Is the choice simply one that the composer decides to make, or is there another reason why one would be better over another?

Answer 1

First, sorry if I’m splitting hairs here but we are talking about diminished 7th chords here and not diminished triads. I mention that because we need the full Ebo7 over D to create the D7b9. It seems like when people (myself included!) say “diminished chord” we are often talking about the 7th variety. I will use the term “diminished chord” as a blanket term covering 7ths here.

As for your two song examples, since we can’t count on lead sheets as accurate representations of a composition I listened to the Fred Astaire version of “A Foggy Day”, which is where the song originated in a 1937 movie. The second chord is a #idim on both A sections.

I was unable to find a soundtrack version of Have You Met Miss Jones (from a musical also in 1937) but early recordings have very different chords in the second bar. Benny Goodman plays a V7/ii. Louis Armstrong plays a ii7-V7/ii and Sinatra plays a #idim so take your pick!

As for your final question, I would say it is the composers choice to make whether to use a diminished chord or a 7b9. That being said, creators of fake books,arrangers and performers often take liberties with compositions for various reasons. As for one being better than the other, that would depend on context and personal opinion to an extent. Are we orchestrating? Arranging? Playing the melody? Improvising over the changes?

Another thing to keep in mind is that diminished chords have several functions. Most commonly it resolves up a half step. Sometimes it resolves to the same root, io-I (i.e. Spring is Here). It can also resolve down a half step. In these other instances the 7b9 chord may not be suitable.

If I am making a harmonic choice, one of my main tests is: Does what I’m doing work well with the melody? If it doesn’t then it goes in the trash. When improvising my note choices for diminished chords tend to be very different than they are for 7b9 chords so that will play a part as well.

As for your title question, I would say a diminished chord voicing will pretty much always work as a substitute for a 7b9 but not necessarily the other way around.

Answer 2

Is the choice simply one that the composer decides to make, or is there another reason why one would be better over another?

I would say it's a decision an arranger or interpreter makes. Accompanying the melody with a D7♭9, E♭dim7, E♭dim, F♯dim, F♯dim7, D7, D or just an F♯ note does not change or spoil the main idea of the composition, IMO. It's still the same song. And if there's a bass player playing a D, all of those chords achieve the same thing - a secondary dominant chord suggesting a Gm as a probable next chord.

Would a composer approve of some of those alternatives? They probably wouldn't give you arrangement royalties, but they would let you play your version as long as they they get their money. The composer, or whoever currently owns the rights.

What might a composer have written about those chords in an original manuscript or definition of what the main idea of the composition is? These being jazz standards, the de-facto "original manuscript" isn't even written by the composer, it's in the minds of everyone who've heard the definitive recordings and countless other versions.

As a guitarist, you're actually supposed to thin down your voicings to leave room for other players, if it's a larger group. A D7♭9 chord contains an E♭dim7 (as well as an F♯dim7 etc.) so by leaving out the D bass note you're doing what a guitarist should do anyway. Just make sure your lowest note isn't too close to bass range.

Answer 3

7♭9 chords must contain an interval of a minor 9th (or larger compound interval that is still equal to 13 mod 12 or 1 mod 12 semitones). This interval is often undesirable due to its resemblance to the dissonant minor 2nd, especially in classical music (although 7♭9 chords are still occasionally found there).

Diminished 7th chords can also be used to modulate smoothly to chromatic mediant keys - e.g. while B°7 normally resolves to C or Cm (in its role as vii°7), its enharmonic re-spellings can all be used to resolve to other chords: D°7 to E♭ or E♭m, F°7 or E♯°7 to G♭ or F♯m, and G♯°7 to A or Am.

Diminished 7th chords can also be used as common-tone diminished 7th chords, which share at least one note (often the root of the second chord) with the chord straight after it. An example of a common-tone diminished 7th chord is the diminished 7th chord in C°7-C (which might be spelled more understandably as F♯°7/C).

Note that 7♭9 chords cannot pull off any of the functions above that diminished 7th chords can pull off.

Plain old diminished chords (i.e. m♭5) may be able to be reinterpreted as inversions of diminished 7th chords with notes missing.

Answer 4

As you said, if there is a bassist then there is no need for a guitar/piano to play the root, i.e., replacing the 7flat9 with the diminished seventh chord that remains when the root is removed sounds fine (or perhaps it sounds even better, with musicians giving each other space).

On the other hand, when there is not a bassist, the diminished chord replacement might still sound OK, but I find that something is definitely lost especially when the original purpose of the 7flat9 is to resolve to the minor chord a fifth below (and this is often the purpose of a 7flat9 chord). Let's pick a famous example from popular music. The opening of "I will survive" by Gloria Gaynor starts with what sounds like several octaves of diminished arpeggio that is in fact anchored by an E bass note that makes the diminished seventh chord into E7flat9 that resolves to the Am chord. The resolution is so strong for (at least) two reasons.

1. Tritone resolution -- The extra tritone that is in the diminished part of the E7flat9 resolves outward to an interval contained in the Am triad (one does not get this with the plain E7 chord).
2. The motion of the chord roots is 5 to 1 as in a perfect cadence.

If the 7flat9 is replaced by a diminished seventh chord then you still have the tritone resolution in item 1, but you lose item 2 by discarding the root.

Answer 5

Some considerations:

1. If you're writing 4-part harmony then you may not have a voice to spare for the root at 5-th note in the chord.
2. Diminished chords often have their own root motion: C-C#o-Dm-D#o-C is a relatively common sequence. It wouldn't sound the same if you artifically put A and B roots on the passing chords.

Answer 6

Dominant 9th and half-diminished chords are interchangeable in some kind of music. If it is '7flat9flat' then it is interchangeable with a diminished seven. Then yes, it might be the choice of the performer or the composer. But don't make a generalisation. This is all style-specific. In case of your question, you can substitute a dominant chord with a diminished chord. Again, it is style-specific and sometimes even composer-specific but it is not irrelevant to tonal concept.