Altered Scale

The altered scale is the seventh mode of the melodic minor scale and the primary tool for playing over altered dominant chords (7♭9, 7♯9, 7♭13, 7♯11, and combinations). It contains every possible alteration of a dominant seventh chord simultaneously — ♭9, ♯9, ♯11, ♭13 — which makes it maximally tense and maximally suited for creating the kind of dissonance that demands resolution to the tonic. Bill Evans, John Coltrane, and virtually every post-bop jazz musician uses it constantly.

Construction

G altered scale (7th mode of A♭ melodic minor):
G  A♭  B♭  C♭  D♭  E♭  F  G

Formula: 1  ♭2  ♭3  ♭4(M3)  ♭5  ♭6  ♭7

Compared to G Mixolydian:
Mixolydian: G A  B  C  D  E  F  G  (unaltered)
Altered:    G A♭ B♭ B  D♭ E♭ F  G  (everything altered)

Both the ♭9 (A♭) and ♯9 (B♭) are present. The ♯11/♭5 (D♭/C#). The ♭13 (E♭). The major 3rd (B, enharmonically C♭) and ♭7 (F) — the identity of the dominant chord — are preserved. This scale provides maximum harmonic tension over a G7 chord while retaining the chord's defining notes.

Parent Scale: Melodic Minor

A♭ melodic minor: A♭ B♭ C♭ D♭ E♭ F G A♭
7th mode (start on G): G A♭ B♭ C♭ D♭ E♭ F G
                       = G altered scale

The altered scale is always the 7th mode of the melodic minor
a half step above the root.
G altered = A♭ melodic minor starting on G.
C altered = D♭ melodic minor starting on C.
F altered = G♭ melodic minor starting on F.

Sound and Use

The altered scale sounds tense, complex, and modern. Over a G7 chord resolving to Cmaj7, the altered scale creates the maximum possible tension before the release. The ♭9 and ♯9 together create a thick dissonance; the ♭13 and ♯11 push away from any stable sound. All of this tension resolves when you land on the C major notes at the resolution.

Classic ii-V-I with altered dominant:
Dm7:       D Dorian or D natural minor
G7alt:     G altered scale (= A♭ melodic minor)
Cmaj7:     C Lydian or C Ionian

The key to using the altered scale is the resolution — you pile up tension over the V7 chord and land cleanly on the tonic. If you don't resolve, the altered sound is just noise. If you do, it sounds sophisticated and intentional.

Chord Voicings for Alt Dominant

G7alt voicings:
Root position (guitar/piano): G B F A♭     (root, 3rd, ♭7, ♭9)
Rootless voicing: B F A♭ E♭ (3rd, ♭7, ♭9, ♭13) — particularly rich
Shell: B F E♭                (3rd, ♭7, ♭13 — sparse but clear)