Altered chord

In music, an altered chord, an example of alteration, is a chord with one or more diatonic notes replaced by, or altered to, a neighboring pitch in the chromatic scale. For example the following progression:

uses an altered IV chord and is an alteration of:

The Ab serves as a leading tone to G.

In jazz music, an altered chord is a dominant chord that has the 5th and the 9th raised or lowered by a single semi-tone. An altered chord may contain any of the following:

• b5
• #5
• b9
• #9

They may be in any combination. It is more common to have one of each, for example an altered 5th and an altered 9th rather than both being altered 5ths (or altered 9ths).

Altered chords are commonly substituted for regular dominant V chords in ii-V-I progressions, most commonly in minor harmony leading to an i7 (minor 7th) chord.

When a chord is explicitly notated as an "alt chord" (e.g. G7alt), it generally implies a chord built off the altered scale (1, b9, #9, 3, #11 (=b5), b13 (=#5), b7), in which all of the above alterations are present. The "alt chord" can be analyzed as a kind of tritone substitution; the alt chord on a given root is the same as the 7#11 chord on the root a tritone away (e.g., G7alt is the same as Db7#11).

Another common alteration is the "b9 chord" (e.g. G7b9), which, when in the context of a dominant resolution to a major tonic, is typically voiced with a natural 13 rather than the b13 of the alt chord. When voiced with a natural 13, jazz musicians typically play the half-step/whole-step diminished scale over the b9 chord (e.g. G, Ab, Bb, B, C#, D, E, F).

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