How to Use the Diminished Scale
The diminished scale can be used over diminished chords, as well as b9, #9, or b5 dominant chords by beginning the scale a half step above the root of the chord.
Here are 2 shapes of the C diminished scale starting on the 3rd and 8th frets:
The diminished scale is symmetrical making there only 3 different scales possible. Starting the scale a minor 3rd above or below the initial root results in the same notes. Basically, the scale repeats every minor 3rd movement. The C, Eb, Gb, and A diminished scales all contain the same notes.
The diminished scale may also be used over b9, #9, or b5 dominant chords by beginning the scale a half step above the root of the chord. For example you can use C# diminished over a C7b9 chord. You may also use C over b9, #9, or b5 chords because the diminished scale contains those 3 altered tones.