True Chromatic Chords
There is, however, a large group of chromatic chords that do not originate through the process of modal interchange and the use of secondary dominants.
Augmented Sixth Chords
To obtain a chromatic chord, one or more of the notes of a diatonic chord needs to be chromatically raised or lowered. When this is done in such a way as to create a chord that belongs to no other key, the result is a true chromatic chord.
The Italian, French, and German Forms of the Augmented Sixth
Chords of the augmented sixth come in three well-known traditional forms. The chord used in is the simplest form, having only three notes, is called the Italian sixth. If we add a perfect fifth above the Ab to create a four-note chord, we get a German sixth chord. If we replace that fifth with an augmented fourth, we get a French sixth chord. All three types of augmented sixth chords perform an identical function.
They function as chromatically enhanced pre-dominant chords, the augmented sixth interval opening out to the dominant root and its octave. Augmented sixth chords most commonly occur on the submediant degree of the minor scale and the flattened submediant degree of the major scale.
Augmented Sixth Chords as Chromatically Altered Dominants
Augmented sixth chords are simply chromatically altered dominant or secondary dominant chords. As such, augmented sixth chords really belong in the same category as chromatically altered dominants. The proof of this lies in the fact that musicians use a great many altered dominant chords that have that characteristic interval of the augmented sixth (or diminished third).
The ability to generate unique harmonic colors and scalar environments is only one feature of these chromatically altered dominant chords. They can also be used to create links between distant keys, and these links can then be used to create chord progressions of great color and variety. A good example of this is the technique known as triton substitution
A Wealth of Chromatically Altered Chords
Complex chords, such as dominant ninths, elevenths, and thirteenths, can be altered in a great variety of ways. The upper chord can thus be construed as being one layer and the lower chord—the foundation—another. This upper layer offers great possibilities for chromatic alteration for the simple reason that the function of a complex six- or seven-note chord is determined by the lower layer. So feasibly, you can introduce chromatic alterations to change the color of the upper layer without interfering with or altering the function of the chord. And these can sometimes give the impression that the lower layer is in one key, the upper layer in another. When this happens, it is called bitonality (or polytonality if it is more than two keys).
Less Traditional Chords
The Tristan Chord
The effect of the Tristan chord involves both the initial chord and the harmonic changes that follow as shown below. Analyzed as a type of seventh chord it would be a minortriad (G-sharp, B, D-sharp) plus a diminished seventh (G-sharp to F-natural) in third inversion (not a common-practice seventh). It progresses to a dominant seventh built on E.
The Mystic Chord
The mystic chord is the creation of Alexander Scriabin (1872–1915) and was used in his Prométhée and is therefore sometimes called the Promethean chord. Scriabin’s mystic chord is a chord built from fourths of various sizes. From the bottom an augmented fourth (C to F-sharp), a diminished fourth (F-sharp to B-flat), another augmented fourth (B-flat to E) and two perfect fourths (E to A and A to D).