An oversight concerning a diminished chord:

[Dark Star]

I am going through years of paperwork & trying to catalog all the notes & lessons & stuff I have accumulated since 1978. I ran across a chord chart from when I was in my early 20s of all the basic chord grips in the key of C. They were all messed up, full of suspensions & "cool" fingerings that don't really flow very well to my ears today.
But I noticed Bdim7 to Cmaj7 like this:
X.2.3.1.3.X
X.3.5.4.5.X
The notes in the first chord are
B-F-Ab-D
but Ab in not in the key of C. I'm sure there is a simple explanation. What is it?
thx

[JakeAcci]

A diminished seventh chord is closely related to a dominant b9 chord

Bdim7 = B D F Ab

if you add a G it is G7b9

if you add a C# it is C#7b9

if you add an E it is E7b9

if you add a Bb it is Bb7b9

In this case, the V7 of C is G7, G7b9 to Cmaj7 would be pretty typical, so Bdim7 to Cmaj7 could be considered just V7 to Imaj7 in C, with a 'rootless' voicing for the V7.

Looking at the issue in a bit of a converse way, any time you have V7b9 (or simply V7 and you'd like the b9 tension) then you can use a voicing for either VIIdim7 IIdim7 IVdim7 or bVIdim7 - all four of those chords contain the same notes, which are the notes of the V7b9 minus the root. COnsidering that we often voice chords without the root anyway, it's all a bit synonymous.

Hope that helps.

[kenbennett]

When harmonizing the C major scale in 4-note chords, the chord built on the 7th degree, B, is Bm7b5, not Bdim7.

[JakeAcci]

Originally Posted by kenbennett

When harmonizing the C major scale in 4-note chords, the chord built on the 7th degree, B, is Bm7b5, not Bdim7.

Similarly, the diatonic 9th on a V7 chord is natural, not flat, yet it is probably just as common to hear V7b9 (in jazz) than it is to hear V9. So I'm not sure what the point is

Also on the same page, Dark Star I don't entirely get your question so maybe I jumped the gun with my generic diminished seventh post. Ab isn't in the key of C, but how often does anything stay completely diatonic to any key in jazz?

[bako]

The tonal area of a key is bigger than just the 7 notes of the major scale.

Secondary dominants introduce a new note but are not considered a modulation.

Key of C

A7---Dm7---(C#)
B7---Em7---(D#,F#)
C7---FMa7---(Bb)
D7---G7-----(F#)
E7---Am7---(G#)

Diminished chords work in a similar way.

C#dim---Dm7---(C#,Bb)
D#dim---Em7---(D#,F#)
Edim-----FMa7---(Bb,Db)
F#dim----G7-----(F#,Eb)
G#dim----Am7---(G#)

The 7 note scale contains the featured notes but the remaining 5 can serve as an alternate supporting cast.

[BigDaddyLoveHandles]

I agree with Jake: G7b9 to CMaj7.

[onetruevibe]

Originally Posted by JakeAcci

A diminished seventh chord is closely related to a dominant b9 chord

Bdim7 = B D F Ab

if you add a G it is G7b9

if you add a C# it is C#7b9

if you add an E it is E7b9

if you add a Bb it is Bb7b9

In this case, the V7 of C is G7, G7b9 to Cmaj7 would be pretty typical, so Bdim7 to Cmaj7 could be considered just V7 to Imaj7 in C, with a 'rootless' voicing for the V7.

Looking at the issue in a bit of a converse way, any time you have V7b9 (or simply V7 and you'd like the b9 tension) then you can use a voicing for either VIIdim7 IIdim7 IVdim7 or bVIdim7 - all four of those chords contain the same notes, which are the notes of the V7b9 minus the root. COnsidering that we often voice chords without the root anyway, it's all a bit synonymous.

Hope that helps.

Originally Posted by bako

The tonal area of a key is bigger than just the 7 notes of the major scale.

Secondary dominants introduce a new note but are not considered a modulation.

Key of C

A7---Dm7---(C#)
B7---Em7---(D#,F#)
C7---FMa7---(Bb)
D7---G7-----(F#)
E7---Am7---(G#)

Diminished chords work in a similar way.

C#dim---Dm7---(C#,Bb)
D#dim---Em7---(D#,F#)
Edim-----FMa7---(Bb,Db)
F#dim----G7-----(F#,Eb)
G#dim----Am7---(G#)

The 7 note scale contains the featured notes but the remaining 5 can serve as an alternate supporting cast.

I am humbled, daily, buy how much you guys know about this stuff. Where did all the knowledge come from? school, years of experience, freakish brilliance, all of the above?

Just trying to manage expectations for myself.

[bako]

I am humbled daily by the seemingly freakish brilliance of musicians I listen to in headphones and at times on the bandstand.
Like many I am more focused on what needs to be improved or learnt next than on what I do know.
If all goes well I will play ever so slightly better tomorrow than today.
The things I do know I learned from recordings, books, people and an obsession with permutations.
Anywhere, everywhere, by any means necessary.

[brwnhornet59]

Originally Posted by bako

I am humbled daily by the seemingly freakish brilliance of musicians I listen to in headphones and at times on the bandstand.

AMEN!

Yeah, in the context of C that was functioning as a rootless G7b9 , IMHO. But as was mentioned, 7b9 opens many other doors.

[wizard3739]

I just finished the first chapter of "Jazzology" and am enjoying going through this book. I took the test at the end of Chapter 1 and got them all right except the last test on Dissonance and Consonance. I missed three of the answers but I'm not certain why my answers were wrong. JonnyPac, Have you gone through this part of the book?

wiz

[JakeAcci]

Originally Posted by onetruevibe

I am humbled, daily, buy how much you guys know about this stuff. Where did all the knowledge come from? school, years of experience, freakish brilliance, all of the above?

Just trying to manage expectations for myself.

I'm flattered, but a lot of things are available in text books. I've taken a lot of classes, private lessons, read books, and experiment daily. I am a guitar teacher for a living and I do try to study and practice as much as possible.