Diminished chords are generally *keyless* in relation to major
scale harmony, but are a part of a harmonic minor scale or minor key
harmony. That is why diminished sounds are used, for example, to create
tension in old silent movie music. They are not usually a key
signature. That is, they do not function as harmony with respect to what is
sometimes called a "tonal center." They are usually substituted for dominant
seventh chords and they most often *connect* other chords generated by major 
or minor scales. They are widely used in jazz to reharmonize simple chord
progressions using the dominant cycle of fourths. As a chord substitution,
notice the similarity between the following chords:

   G7-9    =3D G-B-D-F-AB

Ab dim.7   =3D Ab-B-D-F

The following simple rule:

SUBSTITUTE A DIMINISHED CHORD 1/2 STEP ABOVE THE ROOT OF THE DOMINANT
CHORD IN QUESTION. ie., (A7 =3D Bb dim.7)

Diminished chords in jazz harmony usually function as a substitute
resolve on an altered V chord. A II-V-I progression, for example:

 II  -    V   -    I

Dm7 -    G7    -  Cmaj.

Dm7 -    G7-9   - Cmaj.

Dm7 -  Ab dim.7 - Cmaj.


Diminished chords substitute for a dominant seventh -9 chords.

A diminished seventh chord is also the seventh degree(*) or seventh
diatonic chord generated in a harmonic minor scale:

                                                          (*)
   I            II          III     IV     V     VI       VII         I
Am/maj.7 - B half dim. - Cmaj.7+5 - Dm7 - E7 - Fmaj.7 - G# dim7. -
Am/maj.7

The easiest way to play a regular diminished scale is to take
the 4 notes of the diminished chord in question and play the root (4th
string) then, play a fret below (*) on each of the remaining tones.

Tones of an Ab diminished seventh chord - Ab - B - D - F

             (*)      (*)    (*)      (*)
PLAY:  Ab   (Bb)B    (Db)D   (E)F    (G)Ab

-----------------------------2--2-----5--6--------------------------------
---------------------2--2-------------------------------------------------
------------3--4----------------------------------------------------------
-------6------------------------------------------------------------------
--------------------------------------------------------------------------


On the PC web site go to:

http://wmpub.com/PChtml/alphabet.html
The re-harmonization of the "Alphabet Song"

http://wmpub.com/PChtml/jp9.html
Altered dominant jazz chord resolves

The Joe Pass guitar Style book also has a great section on diminished
chords, p.28


Copyright 2000 White Mountain Publicatons

Pat Cloud