People have "free-for-all" with DIM chor

we dont?

okay :)

Chalmodo

In the key of C , you can also use a C#dim chord to connect a C (I) chord to a DM (IIm) chord. You can also use an G#dim to connect a G (V) chord to an Am (6m).

In popular music, they're actually MORE common than a Bdim.

       www.Guitarforsongwriters.com

Yes.  And the Bdim contains mostly the same notes as a G7 and the fulfil the same function (dominant).  

A lot of rock/pop/folk songs use a bVII chord instead of the viidim (Bb in C).  It works very well, but lacks the tension of the viidim.

well the diminished chords do have an important function really.

easy to move about because all augmented / diminished chords are only really inversions of each other.

im not sure if this is right but i think there is only 3 diminished chords that arent inversions of each other.

or you can take off the root and add a minor 3rd on the end and get a new chord.

i suppose playing a B major would be weird cos of the D# and F#, and B minor is more of a closer idea that will sound ok, beside that F#.

and people do play Bb chords in C, the flattened 7th chord, the Beatles used it LOADS in their psychadelic days

Yes.  And the Bdim contains mostly the same notes as a G7 and the fulfil the same function (dominant).  

A lot of rock/pop/folk songs use a bVII chord instead of the viidim (Bb in C).  It works very well, but lacks the tension of the viidim.

That is rather interesting.  So instead of using the vii dim often a V7 is substituted?  I see the only difference between those two chords is the root note of the V7 is a half step flat of the vii dim.  Still, I know people often play the V chord as a V7 and it works.

Now lets get more into what can be played instead of the vii dim.  Could you guys/gals list some alterations in order of most used to least (for me, say rock music)?  That way I might be able to alter those chords when I don't want to play it.  Actually, I never play dim chords unless a song says to and I see them infrequently.

It was odd I was still laughing that I posted this but it does bother me.  He like the key of F much more than the rest of us.  Cool.  But I feel like "don't play the Bb in every song because you like it."

I just looked on my handy chart and it looks like these diminished chords have the exact same note so are just  inversions of one another:

B = D = F

A = C

E = G

and those notes are both relative to one another

D major / B minor

C major / A minor

G major / E minor

and therefore the chords that come are from the same scale so a B dim will be in Cmaj/Amin and a.. C#dim will be in Dmaj/Bmin etc.

A far more knowledgable guitarist than I am once told me that dim chords are almost never used in major keys.  It's always (99% of the time) the V7 instead.  

If you see a dim chord, it's almost always functioning as the ii in a minor key.

That's what he said.  

Its the dim7 chords that are all inversions of each other.  There are essentially only three of them.

Bdim7 = Ddim7 = Fdim7 = Abdim7  (B-D-F-Ab) (B-D-F-G#)
Cdim7 = Ebdim7 = Gbdim7 = Adim7 (C-Eb-Gb-A) (C-D#-F#-A)
Dbdim7 = Edim7 = Gdim7 = Bbdim7 (Db-E-G-Bb) (C#-E-G-A#)
and they are fingered:

    Bdim7 Cdim7 Edim7
e----1-----2-----3-----------
B----0-----1-----2-----------
G----1-----2-----3-----------
D----0-----1-----2-----------
A----x-----x-----x-----------
E----x-----x-----x-----------

"A far more knowledgable guitarist than I am once told me that dim chords are almost never used in major keys.  It's always (99% of the time) the V7 instead."(from Helgi's post)..........That's a bit of an exaggeration. He's right that dim. chords are not used as V7 chords. BUT, they are frequently used as connector chords in one of two ways: (1) to connect a I chord to a IIm (G,G#dim., Am) or (2) to connect a V chord to a VIm (6m). Songs that come to mind are "Friends in Low Places" (intro, in A:A, A#dim.,Bm) "Wind Beneath My Wings" leading into chorus in G: D, D#dim., Em.

     www.guitarforsongwriters.com

Helgi, you sealed the deal.  I can play diminished chords and my fingers snap to them fairly well but now I see it is not needed.  V7 here I come.

Just to clarify, you won't see a fully diminished 7th chord as ii.  Those will show up as iiº or iiø (not iiº7).  Only viiº7 is a fully diminished 7th chord.  

corbind:  Don't forget about dim7 chords; often times they are needed!  When I played in a blues band for several years, I played dim7 chords all the time!  As garytalley demonstrated, they are used in pop and country as well.

A lot of rock/pop/folk songs use a bVII chord instead of the viidim (Bb in C).  It works very well, but lacks the tension of the viidim.

That would just be borrowing the bVII major chord from the parallel Mixolydian,right?

technically what you said is true.. and not true.

the bVII is used to create a modal sound, not necasserily the mixolydian one... although mixolydian does make sense cos the one accidental is the flattened seventh, which the chord is built on..

but also the Dorian (b3, b7), Aeolian (b3, b6, b7) and Phrygian (b3, b4, b6, b7)

if you get my point, if all you saw was the flattened seventh chord then my guess would be it is aimed to create a mixolydian modal sound.

so really its not "borrowing" from the mixolydian scale its more, used to underline the modal sound under a mixolydian melody.

There's a good substitution for a 7b9 or ° chord.

Look at the notes in D7b9. D F# A C Eb.
Now get rid of the root. It's now an F#°. F# A C Eb.
Invert it and you can get an A°, C° and Eb°.

Because a D7b9 with the root omitted gives an F#°, and that the F#° can be inverted to make other ° chords, we can assume that these inversions have their own 7b9 subs.

In fact, an F7b9 (7b9 sub of A°) has the notes F A C Eb Gb. Omit the root. Look familiar? What about a B7b9? Or an Ab7b9?

This means that not only do we have the tritone sub (b5) when working with 7b9s, but also (potentially) 4 symmetrical substitutions (each root a b3 apart) in total - just like ° chords!

I just farted this out of my brain so feel free to tear it apart.

It also occurs to me that you could replace any "connecting" ° with a ii-V leading into the next chord.

ie | C / C#° / | Dm / / / |
becomes
| C / Em A7 | Dm / / / |

"That's all old hat!" I hear you say. Well, let's apply the good old ° substitutions.

If C#° can give us an Em A7 change, then certainly this same "extrapolation" could apply to other chords. With this, we could make a formula based on the above information and apply it to a ° inversion (but I won't because I'm lazy).

| C / C#° / | Dm / / / |
becomes
| C / E° / | Dm / / / |
becomes
| C / Gm C7 | Dm / / / |

| C / C#° / | Dm / / / |
becomes
| C / G° / | Dm / / / |
becomes
| C / Bbm Eb7 | Dm / / / |

| C / C#° / | Dm / / / |
becomes
| C / A#° / | Dm / / / |
becomes
| C / C#m F#7 | Dm / / / |

Hell, the C#° could just be subbed for a 7b9 chord if you remember what we were saying at the start of this rant.

As always, your mileage may vary. Don't substitute if it clashes with the melody. If in doubt, use your ear.

PS the vii(7)b5 chord mentioned earlier in the thread (found diatonically in a major key) is only half-diminished, so if you were getting any ideas about subbing that then don't bother. Apart from the V7, the vii can also be easily substituted for the ii (for instance, Bm7b5 is simply an inversion of Dm6).