Hi GMC! This is the fourth and last lesson of the series that show how many different scales can we play over a dominant chord.

I've made phrases in A whole-tone scale and A diminished to play over the A7. Like in the previous lessons I play one scale over the A7 then comes the D7 chord (where I play D mixolydian or D major sometimes) and when the A7 comes again I play the other scale that we are learning here.

The chords that plays the rhythm guitar are: A7-D7

A whole-tone  – A -B -C# -D# -F -G -A (this scale add the tensions: 9-b5 or #4- b13; although this scale has no perfect fifth the construction of the dominant chord is based on: root, major 3rd and minor 7th that's all you need to build up a dominant chord)

A diminished half-step, whole-step - A -Bb -C -*Db -Eb -E - F# -G -A (this scale add the tensions: b9-#9-b5 or #4)

*The Db is enharmonic a C#.

Before the lesson begin I want to say a few things about these scales.

They are known as the Symmetrical Scales because the intervalic structures are always the same. The first one has six notes and the second one has eight notes. This “strange” structure requires some note choices. In the hexatonic scale you'll have to put aside one note name (in this case I left the E) and in the diminished scale you'll have to repeat a name (in this case I repeated the E).
These scales are easy to learn because they have very few transpositions.

Whole-tone: C- D- E- F#- G#- A#- C 

Transposition 1: C#- D#- F- G- A- B- C# 

When you start over the D is the same whole-tone scale as the one that starts in C and because this scale is symmetrical doesn't have modes.

Diminished scale half-step.whole-step:  C- Db- Eb- E- F#- G- A- Bb- C 

Diminished scale whole-step.half-step :       Db- Eb- E- F#- G- A- Bb- C- Db

Transposition 1 has-hos: C#- D- E- F- G- Ab- Bb- B- C #

Transposition 1 hos-has:        D- E- F- G- Ab- Bb- B- C #- D

Transposition 2 has-hos: D- Eb- F- Gb- Ab- A- B- C- D

Transposition 2 hos-has:      Eb- F- Gb- Ab- A- B- C- D- Eb

I explain what I just did above.

The diminished scale has only three choices: the one starting in C, the one starting in C# (or Db) and the one that starts on D. The reason this works this way is that if you start the scale over the Eb it will be the same as the one that starts on C and so on. Below the half-step.whole-step scale I put the other diminished scale: whole-step.half-step. But this scale is the same compared to the half-step.whole-step but displaced (that's why I aligned the scales in that way). The result is that you have only three diminished scales to learn because the rest of the transpositions are the same of those three.

The catchy thing is that is not the same a  half-step.whole-step that starts on C compared to a whole-step.half-step that starts on the same note. 

Diminished scale half-step.whole-step:  C- Db- Eb- E- F#- G- A- Bb- C 

Diminished scale whole-step.half-step :  C- D- Eb- F- F#- G#- A- B- C

As you may notice they share a notes (C- Eb- F#- A) The ones that make the diminished seventh chord: C-Eb- Gb- A. So with the whole-step.half-step you cannot make a dominant chord because you don't have the major third and the minor seventh. You could give it a try but it will sound very outside.