In jazz and jazz harmony, the term altered chord, notated as an alt chord (e.g. G7alt Play (help·info)), refers to a dominant chord, "in which neither the fifth nor the ninth appears unaltered". – namely, where the 5th and the 9th are raised or lowered by a single semitone, or omitted. Altered chords are thus constructed using the following notes, some of which may be omitted:
♭5 and/or ♯5
♭9 and/or ♯9
Altered chords may include both a flatted and sharped form of the altered fifth or ninth, e.g. G7♭5♯5♭9; however, it is more common to use only one such alteration per tone, e.g. G7♭5♭9, G7♭5♯9, G7♯5♭9, 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:
♯9 (=♯2 or ♭3)
♯11 (=♯4 or ♭5)
Altered chords can be analyzed as a kind of tritone substitution (♭5 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 D♭7♯11 Play (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 (tonic minor 7th) chord.
More generally in jazz, the terms altered chord and altered tone also refer to the family of chords that involve ♭9 and ♭5 voicing, as well as to certain other chords with related ambiguous harmony. Thus the "7♭9 chord" (e.g. G7♭9) is used in the context of a dominant resolution to a major tonic, which is typically voiced with a ♮13 rather than the ♭13 of the alt chord. When voiced with a ♮13, jazz musicians typically play the half-step/whole-step diminished scale over the ♭9 chord (e.g. G, A♭, B♭, B, C♯, D, E, F over G7♭9).
Note that in chord substitution and comping, a 7♭9 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 A♭-dim (A♭-C♭-E = G♯-B-D), D-dim (D-F-A♭), B-dim (B-D-F), or F-dim (F-A♭-C♭ = F-G♯-B)), a G7♭9 is often played instead. G7♭9 (G-B-D-F-♭A) contains the same notes as any of these diminished chords with an added G root.