One of the factors that gives chromatic chords such a strong functional use is the presence of tendency tones. These tendency tones resolve by half-step, and create strong, functional harmonic flow. Since the dominant (V) chord is has the strongest "pull" toward tonic, most chromatic harmonies serve as predominant harmonies and resolve to the dominant. Common diatonic predominant chords are the ii (or ii7) and IV chords.
Throughout the common practice period, composers found that by combining the lowered scale degree 6 from borrowed chords and the raised scale degree 4 from the V/V chord, a strong, functional predominant harmony could be created. Typically, the lowered scale degree 6 is voiced on the bottom, and the raised scale degree 4 is on top, resulting in the unusual interval of an augmented sixth (Ab up to F# in the example below). Hence, this category of chords is known as augmented sixth chords. Although these chords have names which refer to various nations, they were freely used in music of all nationalities.
All Augmented sixth chords contain a lowered (if in Major keys) scale degree 6, tonic, and a raised scale degree 4, as shown above. In each case, the chromatically altered tones resolve by half-step to the dominant (scale degree 5).There are three basic types of augmented sixth chords:
Italian Augmented Sixth
The Italian Augmented Sixth chord (notated as It+6)has the three above listed notes and doubles the tonic in four-part voicing. Voice leading is the same for both major and minor modes. It may resolve to either a V or a tonic 6/4 chord (which then normally resolves to a V chord).
French Augmented Sixth
A French Augmented Sixth chord (notated as Fr+6) adds scale degree 2 to the above three pitches. Voice leading is the same for both major and minor modes. Notice how the voice leading smoothly resolves to either a V or tonic 6/4 chord:
German Augmented Sixth
The third type of augmented sixth chord has two different spellings: one for major keys and one for minor keys. In minor keys, the normal German Augmented Sixth (notated as Ger+6) is used. In this chord, a minor third above tonic (scale degree 3) is added to the three basic tones. As this chord resolves, notice how there are two common tones.
In major keys, the chord is often respelled enharmonically to allow for smoother voice leading. The lowered scale degree 3 is respelled as a raised scale degree 2, forming the strange interval of a doubly-augmented fourth above the lowered scale degree 6. For this reason it is often referred to as a ++4 or "Doubly-Augmented Fourth" chord. (It may also be referred to as an "enharmonicly spelled German Augmented Sixth" chord). Take careful note of the voice-leading: the raised scale degree 2 always resolves up by half-step to scale degree 3 as the other tendency tones resolve out to the dominant, forming a I 6/4 chord. Neither form of the German Augmented Sixth chord resolves to V, since this would result in parallel fifths or unresolved tendency tones.
The Neapolitan Chord
The Neapolitan Chord is based on a ii diminished chord from minor, but may be commonly found in both Major and minor keys. It uses the lowered scale degree 6 (from the minor mode) and a lowered scale degree 2, and is almost always found in first inversion. It is analyzed using either the symbol bII6 or more commonly: N6. The Neapolitan chord resolves to either a V or a tonic 6/4 chord. (Note: When resolving to the tonic 6/4 chord, the Neapolitan chord is voiced so that the fifth of the chord is above the root in order to avoid parallel fifths in the resolution. The parallel fourths in this voicing are acceptable.) Also note the voice leading when resolving to the V chord: the augmented second is acceptable.
Chromatic Mediant and Coloristic Chord Usage
All of the chords in this section were not randomly "invented" by theorists, but rather, came about by experimentation by composers who used these harmonies frequently and consistently enough for their inclusion in our discussion of tonal theory.
A Chromatic Mediant relationship exists between any two major chords whose roots are a third apart. This not only includes the familiar bIII and bVI borrowed chords, but also a major III and VI chord.
These chords usually occur in root position and after a tonic triad (in major keys) and normally resolve back to tonic, to the dominant, or continue in root movement by successive thirds.
Notice that these chords do not appear to have a strong functional resolution, but rather, provide a colorful "other-worldlyness" to the music. In analyzing the music of the 19th and 20th centuries, you will often encounter chords that do not seem to function according to the normal "rules" of tonal theory, but rather are used for the "color" they add to a progression. When these chords are encountered, it is important to remember that music is an art, and it is often the moments when composers write passages "outside the norm" that music becomes its most expressive. Richard Wagner (1813-1883) used a new set of pitches in his opera Tristan und Isolde that was very shocking at first, but was later favored and used by other composers. It is now referred to as the Tristan Chord:
This section does not present every possible chromatic harmony, but does list those most commonly encountered. Composers today are still "inventing" new chords, some of which may find their way into a theory text in the future. In any case, keep an open mind and creative approach to this ever-changing and expanding artform of music.