The Dominant scale (Dom7) has the VII degree of the Major scale diminished by a half note. It is also the V mode of the Major scale, the Mixolydian mode. The Dominant scale has therefore no leading tone anymore and sounds less happy, but has more suspense.
The Dom7 scale is mostly played over seventh chords and is therefore in the II-V-I progression.
If we expand the triad with a third a seventh chord is created
The dominant seventh chord consists of the triad on the V degree of the major scale and the scale’s seventh.
Dominant seventh chord in C major is as a result G B D F
The dominant seventh chord that is built from the root C (C E G Bb) belongs to F major scale.
You can find more information about seventh chords under the Chord-Clock topic.
Dominant 7th Suspended 4th Chord
For the dominant 7th suspended 4th chord the IV degree replaces the III degree. If the dominant7 scale is used for it you should use the note of the III degree only as a passing tone. For C7sus4 chord the Dom7-BeBop scale or the Bb-major pentatonic scale (Bb C D F G Bb) fit as well.
The II – V – I Progression
If we build on the second, fifth and first degree of a major scale seventh chords, by adding thirds, we get again the three types of the seventh chords mentioned.
This progression is widely used (not only in Jazz!), because it contains one of the strongest resolutions. The intervals between the roots are for this progression perfect fourths (ascending) or perfect fifths (descending). Under the topic Chord-Clock you will find further information about this progression.
The II-V- I Progression in Major Keys
In the major key we have following chord types, chords and suitable scales. The scales deal with the II, V and I mode of the major scale.
For recollection the involved modes of the major scale
Over the entire II-V-I progression you can play the notes of the I degree.
To have everything sound a little bit more exciting you have three possibilities:
1. Scale substitution (Play other scales that fit the chords)
2. Altering of the V-dominant chord.
3. Tritonus substitution.
3. Tritonus Substitution
Tritonus, lat./greek = Three-Note-Step = Augmented fourth
The G7 chord is substituted by the Db7 chord, which is a Tritonus away.
Here are the two chords for comparison.
As you can see, the characteristic sounding chord notes, the third and the seventh are contained in both chords and have just exchanged their place.
The Db7 chord is very similar to the G7b9b5 chord (refer to altering of the dominant chord).
Following scales fit the new Db7 chord :