I've been spending some time working on exercises and scale patterns related to the diminished scales. You've heard of major and minor scales? Well, those are good to know for sure, and any good jazz player should be able to pull a few modes out of the hat--particularly Dorian modes for minor chord passages, Mixolydian for dominant chord passages, and even a Lydian scale mode for #11 passages. (And really, once you learn your major/minor scales, it's just a matter of displacing your starting point and you are instantly in a mode!)
Still with me?
The diminished scale is something I've always been scared of. Instead of seven notes in the scale, like all the scales listed above, the diminished scales have eight notes. This is odd because it is an adjustment for fingering (I still haven't found good fingering for the scales) but it's also an aural adjustment. The octave doesn't come when you expect it to. There's always one more note to go!
The diminished scale is based on an alternated whole step/half step pattern. Because of the way Western music has evolved over the centuries, this means there are only three of these scales before they start to repeat. Here is what they look like.
The reason I want to get to know them is because I have always struggled with how to handle long passages with prolonged diminished chords. More on that later. But I have recently learned that diminished scales are particularly useful over altered dominants. Chords with sharp 9ths, flat 9ths, sharp 11ths, and 13ths can all be embellished by choosing notes from the right diminished scale. Here is an example of how the notes line up over a G dominant harmony, and how each of the notes of the diminished scale play out functionally.
So back to the long prolonged chords of diminished harmony. Duke Ellington's "Caravan" is a good example of this. You can hear a 30 second clip of it on iTunes here. You can see that the entire A section is a prolonged Gdim7 chord. In F minor, this functions as the vii(dim) chord, or functions as dominant. Can you tell which scale from my first example above the melodic material is based on?