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In music, an altered chord, an example of alteration (see below), is a chord with one or more diatonic notes replaced by, or altered to, a neighboring pitch in the chromatic scale. For example the chord progression on the left uses four unaltered chords:
The progression on the right uses an altered IV chord and is an alteration of the previous progression. The A♭ in the altered chord serves as a leading tone to G, which is the root of the next chord.
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 on the altered scale, which includes all the alterations shown in the chord elements above:
- ♭9 (=♭2)
- ♯9 (=♯2 or ♭3)
- ♯11 (=♯4 or ♭5)
- ♭13 (=♯5)
Because they don't have natural fifths, 7alt chords support tritone substitution (♭5 substitution). Thus the 7alt chord on a given root can be substituted with the 13♯11 chord on the root a tritone away (e.g., G7alt is the same as D♭13♯11 Play (help·info)).
Altered chords are commonly substituted for regular dominant V chords on the same root 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.
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In music, alteration, an example of chromaticism, is the use of a neighboring pitch in the chromatic scale in place of its diatonic neighbor such as in an altered chord. This should not be confused with borrowing (as in borrowed chord), in which pitches or chords from the parallel key are used in place of those of the original key. Altered notes may be used as leading tones to emphasize their diatonic neighbors. Contrast with chord extension: "Whereas chord extension generally involves adding notes that are logically implied, chord alteration involves changing some of the typical notes. This is usually done on dominant chords, and the four alterations that are commonly used are the ♭5, ♯5, ♭9 and ♯9. Using one (or more) of these notes in a resolving dominant chord greatly increases the bite in the chord and therefore the power of the resolution." "The more tension, the more powerful the resolution...we can pile that tension on to make the resolution really spectacular."
The ♭9 chord is recommended for resolution to minor chords, for example VI7 to ii (G7♭9 to Cm7) in the I-vi-ii-V turnaround. The ♯9 chord is also known as the Purple Haze chord, is most often notated with the enharmonic equivalent ♭3, and is thus used with the blues. The 5 in a ♭5 chord is enharmonically equivalent to a ♯4 or ♯11, but the eleventh chord includes the ♮5 while in the flat chord it is replaced. The ♯5 chord is enharmonically equivalent to a ♭13, does not include the ♮5, and is more common than the ♭13 chord. Both the flat and sharp fifth resolve nicely to the natural ninth.
In jazz, chromatic alteration is either the addition of "notes which are not diatonic to the given scale" or "the expansion of any given [chord] progression by adding extra nondiatonic chords". For example, "A C major scale with an added D♯ note, for instance, is a chromatically altered scale" while, "one bar of Cmaj7 moving to Fmaj7 in the next bar can be chromatically altered by adding the ii and V of Fmaj7 on the second two beats of bar" one. Techniques include the ii-V-I turnaround, as well as movement by half-step or minor third.
For example, an altered dominant or V chord may be G♭-B-D♯ (♭5 and ♯9).
Altered seventh chord
An altered seventh chord is a seventh chord with one, or all, of its factors raised or lowered by a semitone (altered), for example the augmented seventh chord (7+ or 7+5) featuring a raised fifth (C7+5: CEG♯B♭). Most likely the fifth, then the ninth, then the thirteenth.
In classical music, the raised fifth is more common than the lowered fifth, which in a dominant chord adds Phrygian flavor through the introduction of ♭. (for example, in C the dominant is G, its fifth is D, the second scale degree)
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