On the use of diminished chords

As I get into more complicated blues progressions, I'm often seeing diminished 7th chords used as passing chords. But I don't have a solid understanding of the "rules" that govern such substitutions.

For example, I've seen a blues progression that substitutes the Idim7 for the IV in the 6th bar (the 2nd bar of the IV). This works because Idim7 leads or resolves to the I7. Can you use it anytime you're heading towards the I? Can you go this with other chords (i.e., IVdim7 to IV)?

Another common one is in Freddie King's "The Stumble". The changes during the "break" section are E-E-A-Bbdim7. The chromatic bassline there sounds pretty familiar to me from other tunes as well. But what's that chord substituting for (if anything)?

Or in "The Stumble", should A-Bbdim7 really be written as "A-A/Bb"? (I.e., stay on A but with a Bb bass?) That's the only note that would change in a Bbdim7, am I correct?

Any help on understanding "the weirdest triad"? Augmented I understand... diminished, not so much...

Thanks!

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Originally Posted by Jesse S.

Any help on understanding "the weirdest triad"? Augmented I understand... diminished, not so much...

Thanks!

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What's weird about it? All minor 3rd intervals.

Here's something you might be aware of. On any dim7 chord, if you lower any note 1/2 step you have a 7th chord. The root is the note just moved from the dim7. Raising any note a 1/2 step will give you a minor 7 flat 5. (the raised note is the 7th) or a 9th chord (no root, the raised note is the 9th).

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Originally Posted by JAM

What's weird about it? All minor 3rd intervals.

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That much I understand - how to build it. It's how to *use* it that hasn't sunk in yet.

You've asked about "rules" that govern when the diminished chord can be used. You seem the have a better handle on this than you realize. The examples you give all work, but I'm not sure there is an exact 'rule' on too many things musical. I have a fair amount of experience both as a student and teacher, but at some point it has to come down to what sounds right and what doesnt. Charlie Parker once said "Master your instrument, Master the music, and then forget all that sh** and just play.”

Maybe to understand the progression in "Stumble" better, look at the dim7 chord as Edim7 - Edim7,Gdim7, Bbdim7 andC#dim7 are the same chords in different inversions.

Ok, after doing a bit more thinking about it, lemme ask this:

When you find diminished chords in the music of groups like The Beatles, Freddie King, or other musicians that weren't into jazz or schooled in music theory, do you think those are "intentional" diminished chords, or just side-effects from a chromatic bassline movement, etc.?

In other words, when a song goes from C to C#dim to D, it's likely that this is really just a walking bassline over a C chord, right?

I never learned diminished chords as distinct chord shapes when I was learning my "cowboy chords". Just wondering if that experience is common or if it's unusual.

As a general rule, in a major progression, the 7th will be a diminished chord.

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Originally Posted by Offshore Angler

As a general rule, in a major progression, the 7th will be a diminished chord.

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Only if you harmonize the scale as triads, using the otherwise the chord formed from the 7th tone in the scale is a minor 7th flat5 (sometimes called a half-diminished)

Jesse, in the example you give (moving from C to D), the diminshed chord is probably a result of a gospel bassline, just as you describe, and not "intentional" (I'm assuming by that you mean 'the songwriter knew they were chosing a diminshed chord and had some kind of compositional reason for doing it').

But there are so many examples from the Beatles (especially in Harrison's writing) where the diminished chord is clearly chosen to either build tension or allow an unusual melodic thing to happen... and in those cases, I think you'd have to call it intentional.

I think, in early blues the guys just found them chords or they were shown to them but they didn't know that they would be called "diminished" - they just liked the tension they provided. I don't know if this helps you any further, but here goes:
let's take an Adim chord - it notes are: F#-C-D#-A
if you move any one of these notes down a half step, you'll have a dominant 7th chord, let's see:
F# down to F: F7
C down to B: B7
D# down to D: D7
A down to G#: G#7

This also works the other way around: if you move any set of 3 notes up a half step, you'll also have a dominant 7th, let's see:

F#,C and D# up to G,C# and E, you leave the A: A7
F#,D# and A up to G,E and Bb, leave the C: C7
C,D# and A up to C#,E and Bb, leave the F#: F#7
F#,C and A up to G,C# and Bb, leave the D#: D#7

So, moving down one note you'll land on the dominant7 of the note you moved, moving up three of the notes you'll land on the dominant 7 of the note you didn't move.
If you play around with these moves you might find a lot of applications were to use them. It's a matter of tension and release.

Depending on what you want to refer as the 'root' moving any note from a diminshed chord a half -step up yields a 9th, minor7flat5 or minor 6th. Example E diminished - E Bb Db G, move the Db to D, now the chord is Emin7 b5, Gmin6 or C9 (no C root). Any Diminished is also a 7th flat9. Fdiminished =G7flat9 (no root) So within just a few frets you have a whole bunch of chords.