I've just come across a way of changing key which I don't think I've seen mentioned here before (although please correct me if I'm wrong).

If you're using a diminished chord, it shares its notes with 3 other diminished chords, and you could move to the root note of the key of either of those and it will sound fine.

For example, if you're playing in the key of C major, and you choose to play a Bdim, you could use it to go to C, but you could also play Eb, or F#, or A, and the chord will sound like it fits, and from there you could play on in that key without sounding "out".

For anyone who doesn't understand why, the explanation is that the full diminished chord uses 4 notes which are each 3 semitones apart (for example B, D, F, Ab). This means that the chord could take its name from any one of these possibilities. For example, this means that you could play the same chord as the Bdim in C major as you would for the Ddim in Eb major.
the chord you are speaking of is a diminished 7th using the intervals 1 b3 b5 bb7

and the Bchord should be diminished to fit in the key of Cmaj the progression goes

CMAJ Dmin Emin FMAJ GMAJ Amin Bdim
  I  ii   iii   IV   V   vi   vii0 

EDIT: a regular diminished chord uses the intervals 1 b3 b5 a min chord uses 1 b3 5 and a maj 1 3 5
song stuck in my head today

Last edited by lbc_sublime at Apr 10, 2008,
I think progressing from Bdim to any of the chords you mentioned except for A sound out of place :/
"Battle not with monsters, lest ye become a monster, and if you gaze into the abyss, your eyes will get sore after a while."
^ well it also depends on the chord that comes before the dim chord too. i think he left that part out.