ALTERED SCALE LESSON
The altered scale (often abbreviated to “C7alt.”) can be played over dominant 7 chords, because it contains a major 3rd and minor 7. It’s called the ‘altered’ scale because every extended harmony note that can be altered (9, 11, 13) has been altered (b9, #9, #11, b13):
The only notes that remain unaltered are the root, 3rd and 7th, which need to stay natural to fit with the dominant 7 harmony.
All these alterations are quite a mind-full, so most musicians simplify the brain work and just think of C altered scale as Db melodic minor (the melodic minor scale is a minor scale with a major 6th and 7th):
So when you see an altered chord symbol on a chord sheet (or just a dominant 7 chord that you want to play altered scale over), get into the habit of playing the ‘melodic minor scale’ building off the flat 2nd of the chord’s root:
So when you see ‘G7′ – play Ab melodic minor scale (Ab Bb Cb Db Eb F G)
When you see ‘D7’ – play Eb melodic minor scale (Eb F Gb Ab Bb C D)
When you see ‘B7’ – play C melodic minor scale (C D Eb F G A B)
But this can still be a lot of brain work, so you can simplify things even further and just aim to play the first 5 notes of the minor scale built off the flat 2nd – so over B7 just aim to play from C D Eb F G.