What's that you say?  How can this be?  (I just got my head wrapped around this, so bear with me....)

Lets take a look at what I'm talking about first....  A diminished 7th interval relationship is I  bIII bV bb7.  That's a "double flat" 7th, or as we humans like to call it, the major 6th.

Lets use Bdim7 as our example.

The notes in Bdim7 are B D F Ab.

Now lets take a look at the notes in the dim7 chords for all the other notes in Bdim7.

Ddim7 is D F Ab B

Fdim7 is F Ab B D

Abdim7 is Ab F B D 

So that's four chords that all share the *exact same notes.*  So what that means is if you are fretting Bdim7, and the next chord is supposed to be a Ddim7, don't move.  You're already playing it.

The other great thing about these chords is that they are eminently movable.  You can slide up the neck but a few frets, and be playing a new voicing of the same chord, unlike the major and minor triads we are all familiar with.  Here's an example.  B/D/F/Abdim7 in 4 places on the neck.   All the same notes, all the same intervals.

So how can that be?  It's because the intervals between any two consecutive notes in the chord are all the same, minor 3rds.  If you keep stacking minor 3rds on top of it, you won't get any new notes, as you would if you were using major or minor triads.  There is no such animal as a "diminished 9th."  All that would do would be to add the root back onto the chord.   In this way, the chord is "symmetrical."

So where can you use these nifty little chord shapes?  They go well over minor keys, so long as the key is one of the notes in the chord.

Handy!