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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 ^[1]:

The next progression uses an altered IV chord and is an alteration of ^[1]:

The Ab serves as a leading tone to G.

Jazz

In jazz and jazz harmony, the term altered chord refers to a dominant chord "in which neither the fifth nor the ninth appears unaltered" and which thus "contains b5 &/or #5, and b9 &/or #9".^[2] – in other words, where the 5th and the 9th are raised or lowered by a single semi-tone, or omitted. This chord is notated as an "alt chord" (e.g. G7alt Play (help·info)).

Altered chords are thus constructed using the following notes, some of which may be omitted:

• root
• 3
• b5 and/or #5
• b7
• b9 and/or #9

The chord may thus include both a flatted and sharped form of the altered fifth or ninth, e.g. G7b5#5b9; however, it is more common to use only one such alteration per tone, e.g. G7b5b9, G7b5#9, G7#5b9, or G7#5#9.

The choice of inversion, or the omission of certain tones within the chord (e.g. omitting the root, common in guitar harmony), can lead to many different possible colorings, substitutions, and enharmonic equivalents. Altered chords are ambiguous harmonically, and may play a variety of roles, depending on such factors as voicing, modulation, and voice leading.

The altered chord's harmony is built off the altered scale, which includes all the alterations shown in the chord elements above:

• root
• b9 (=b2)
• #9 (=#2)
• #11 (=b5)
• b13 (=#5)
• b7

The "alt chord" can be analyzed as a kind of tritone substitution (b5 substitution). Thus 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 Play (help·info)).

Play altered chord with flat 5th, 7th, and 9th (help·info)

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

More generally in jazz, the terms altered chord and altered tone also refer to the family of chords that involve b9 and b5 voicing, as well as to certain other chords with related ambiguous harmony. Thus the "7b9 chord" (e.g. G7b9) is used in the context of a dominant resolution to a major tonic, which 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). (Note that in chord substitution and comping, a 7b9 is often used to replace a diminished chord, for which it may be the more "correct" substitution due to its incorporation of an appropriate root tone. Thus, in a progression where a diminished chord is written in place of a G7 chord, i.e. where the dominant chord is replaced by an Ab-dim, D-dim, B-dim, or F-dim, a G7b9 is often played instead. G7b9 contains the same notes as any of these diminished chords with an added G root.)

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