In a song I am learning, there is a section of the song that has the chord progression G#m | A#m7b5 | B6 I sometimes get the fingerings wrong and use dimished 7th chords on guitar instead of m7b5 chords and in this case I actually think the dimished7th chord sounds good too. So I wanted to ask, what is the difference functionally between the two? They both seem to work in most situations as leading tone chords to a tonic chord one semitone above (B in this case).
In this case it's a "safe" substitution, because the pitches are within the key (
G# minor) and have contextually appropriate functions within the key.
A#dim7 differ only in the presence of
G, respectively, both of which are present in
G# minor. (Technically
Fx, but the distinction isn't important in this context.) The voice-leading works nicely in either case:
X: 1 T: half dim vs. full dim K: G# minor M: none L: 1/1 "G#min"[GBd] "A#min7b5""inverted"[GAce] "B6""inverted"[GB^df] | "G#min"[GBd] "A#dim7""inverted"[=GAce] "B6""inverted"[^GBdf] |
Some points of interest:
- In both cases, there is an upward progression of triads:
G#min A#dim B. In the
min7b5case, the progression occurs over a
G#pedal tone; whereas, in the
Gserves as a leading tone.
Gmin7contain exactly the same pitches, which allows for flexibility of interpretation.
dim7case allows for a descending chromatic line of
G# G F#.
min7b5 is most common as the
ii chord in a minor key. The
dim7 chord appears most frequently as a
vii chord in major or minor.
G#min A#[min7b5 or dim7] B6, allowing for the interpretation
G#min A#[min7b5 or dim7] G#min7, can be interpreted as:
i ii III
i vii i
i vii III
All of which are viable according to the precepts of functional harmony.
Do m7b5 chords function the same as dim7 chords?
If talking about function, it seems better to not use jazz chord symbols and switch to using Roman numeral analysis (RNA.)
The two chord types are half-diminished seventh and diminished seventh.
The diminished seventh chord is easier to describe first. Usually it is rooted on the leading tone or a secondary leading tone. Its RNA symbols is
viio7. Relative to a tonic of
C the chord would be spelled
B♮ D F A♭ (I used the natural to show that in
C minor you would raise the
B♭ of the key signature.) The chord's basic function is a dominant and in that regard some people conceive of the chord as a dominant flat ninth chord with the root omitted.
The half diminished seventh chord is a bit more complicated. In major keys it is the diatonic seventh chord built on the leading tone. In
C major it would be
B♮ D F A♮ and the RNA symbol would be
viiø7. If that chord were used in a major key, its function would be a dominant. But, the chord is actually much more common in minor keys where it is the diatonic chord build on the second scale degree. In
C minor is would be
D F A♭ C and its RNA would be
iiø7. The function of
iiø7 is a subdominant.
The typical function of these chords is not the same.
viio7 is a dominant.
iiø7 is a subdominant.
Those roles also apply to basic jazz harmony.
The diminished seventh chord can appear as a dominant or as a passing chord. The song Bewitched uses the chord both ways. It starts with
C C♯dim7 Dm7 or in RNA
C: I viio7/ii ii7.
The half diminished seventh chord appears in minor, like
iim7♭5 V7alt im6. Autumn Leaves is an example.
The basic triad (which is what usually gives the main "color" to the chord) is the same, and being a diminshed fifth it creates a lots of tension towards the next one - usually a half tone above, and usually the tonic.
They are similar and often interchangeable. While they are not diatonic on both major and minor mode, they can "exist" in those modes:
- in minor mode, the root and seventh of m7b5 are the seventh and sixth melodic degrees ("augmented");
- in major mode, the seventh of dim7 is the harmonic ("lowered") sixth;
The lowered sixth "sounds good" in both modes because it creates a big tension that leads to the fifth, and also due to the fact that it creates another tritone with the third of the chord.
The m7b5 chord is usually considered less powerful than the dim7, as the seventh has less tension towards any resolution, but it can still create interesting effects. The choice depends on the composer/arranger/performer, but it obviously should not collide with any other voice (the melody) and should consider the overall harmony and current harmonic progression.
Taking Bm7-5 vs Bdim7 as examples. If you want to see the chords as working in the same roles as simpler more common chords in the key of A minor:
- Bm7-5 (alias B half-diminished alias Dm6/B) works roughly as a substitute for Dm, so it works as Am's subdominant. Bm7-5 contains all the notes of a Dm.
- Bdim7 (alias B fully diminished) works roughly as a substitute for E7, so it works as Am's dominant. This is because of the D and G# notes.
But as you already noticed, they're quite similar and the difference isn't very big. The functionality of a chord comes as a sum of its parts and the chords share 75% of the same notes.
vii7b5 = (me),se,te,re => V7 without root tone
VIIdim7 = (me),se,te,re,fa => Vb9
me or mi in brackets as it can be considered as dropped root tone of the dom7 respectively dom7b9.
Critical point to decide which chord will better fit is the melody: if it contains the b9 (in F#,A#,C#,E,G) this would be G - then the A#dim7 sounds better, if there is a G# you play the 7b5 chord:A#m7b5
The tritone A♯ and E resolves nicely to B and D♯. Other notes in the chords are less important.
That tritone is found in F♯7 (the standard dominant of B major). It's also found in A♯m7♭5 and in A♯dim7. (Also in C7, if we want to play with ♭5 substitutions.) So they'll all resolve in a nice functionally harmonic way to B major.
There are other ways to approach B major. You can 'plane' up from A♯ major or A major. Don't struggle to analyse this as functional - being the same chord shape and being adjacent are sufficient.
So does this mean a major triad, a dom7 chord, a dim7 and a half-dim7 all 'function the same'? No, that's far too general a statement.
While m7♭5 chords and dim7 chords are fairly interchangeable as dominant-function chords (don't interchange them if the melody note is the 7th), the dim7 chord is a much better pivot chord between keys than the m7♭5 chord, as any given dim7 chord is an enharmonic re-spelling of 3 other dim7 chords, while a m7♭5 chord cannot be enharmonically re-spelled into any other m7♭5 chords. When a dim7 chord is acting as a pivot chord, interchanging it is unwise.
I don't have much to add to other posts concerning the analysis, however I'd like to point another point. In some cases indeed either diminished and half-diminished chords might provide good harmonization of a melody, however they sound substantially different. If you play one or another randomly, due to mistakes, it might be very confusing for the rest of the band. It is very different from substituting e.g. Cmaj7 with C6, which can be innocent 90% of times.
Do m7ba and Do dim7 are exactly the same chord:
Do m7b5 = Do dim7 Do (root) Mib (minor third) Solb (flat fifth) Sib (seventh) La# m7b5 = La# dim7 La# (root) Do (minor third) Mi (flat fifth) (perfect fifth would be Mi#) Sol# (seventh) (mjor seventh would be Sol##)
Anyway, it sounds weird to call A# dim7 and Gm#, are you sure it wouldn't be Abm and Bb dim7 instead?
What geometry for both chords are you using on guitar?