This family of vertical sonorities includes all types of chromatically altered chords that precede the dominant (V) function in tonal music. They typically progress to the dominant, but may be used in other progressions or retrogressions. A borrowed subdominant minor chord followed by the dominant harmony in a major key is technically an altered pre-dominant chord. A secondary dominant, or V/V, would also qualify. But most often the term refers to Neapolitan and Augmented 6th chords.
This chord is simply a major triad built on the lowered second scale degree. It is typically found in a minor key, usually in first inversion. It is analyzed with the symbol “N”, or “N6”. It may also be identified as “♭II”, or “flat two.” It has a striking effect aurally, and is quite a jolt when used in a major key, with both the 2nd and 6th lowered at once. An easy way to spell this triad is to begin with a minor subdominant chord. The 5th of the iv chord is the tonic of the key, and you can create a Neapolitan sixth by replacing the tonic with the lowered 2nd scale degree. This process puts it in first inversion.
The name is derived from 18th century opera composers working in the area of Naples. They did not invent the chord, but the term Neapolitan and the abbreviation stuck.
In four-part Baroque and Classical period music, the third of the N6 chord in the bass was doubled. This altered 2nd is frequently found in the soprano voice, and it resolves downward to the leading tone when followed by the V chord. This creates the interval of an augmented 2nd, which is normally avoided in traditional part writing. It resolves by half-step to the tonic when followed by the tonic chord in second inversion, or to the V chord in third inversion. As in most voice-leading situations, lowered scale degrees generally resolve downward. A lowered 2nd scale degree is the clue to analyzing this chord.
There are three common varieties of this chord, but they all share several characteristics. All three most often function as a pre-dominant chord. Here is a simple way to identify the augmented sixth interval:
It is a linear approach to the doubled fifth scale degree by half-step, in contrary motion.
This chord is most often found in a minor key, in which the diatonic minor 6th scale degree is the bass note. The #4 scale degree of the key is added to create the augmented 6th interval, which is an enharmonically spelled m7th above the bass. The raised 4th is the best clue to identifying any augmented sixth chord. Other chromatic chords with the #4 are V/V and V/iii in major keys, and viio/V in major or minor keys. However, these will not contain a minor 6th scale degree.
To create an Italian, German, or French augmented sixth chord in four voices, add the notes specified below to the augmented sixth interval.
It+6 – Add the tonic pitch of the key, which is doubled. (le, do, fi)
Fr+6 – Add the tonic pitch and the 2nd scale degree of the key. ((le, do, fi, re)
Ger+6 – Add the tonic pitch and the minor 3rd of the key. (le, do, fi, me)
The Ger+6 is enharmonically equivalent to a Mm7th chord built on the minor 6th scale degree. Take away the 5th of the Mm7th chord and it becomes an It+6, which has just three notes. Lower the 5th of the Mm7th chord, spelled as a #4th and you have a Fr+6 chord.
When voice-leading the augmented sixth chords, resolve each tone according to its tendency. The minor 6th and raised 4th resolve outwards, and common tones are retained. When the Ger+6 resolves to the tonic 6/4, two common tones are present between the chords. When resolving the Fr+6 chord to V, the 2nd scale degree is a common tone with the fifth of the dominant chord. These augmented sixth chords are best considered as linear constructs, rather than vertical harmonies in inversion. Some theorists in the past have analyzed the It+6 with the Roman Numeral ♯iv6, but this practice is not common.
When used in major modes, the minor 3rd of the Ger+6 may be enharmonically respelled as #2, which resolves upwards to ♮3. Spelled this way, it is referred to as a doubly-augmented sixth chord. The Ger+6 is enharmonically equivalent to the V7/N, a secondary dominant to the Neapolitan chord. In the key of C, this provides a convenient pivot chord for a chromatic modulation to D♭. In Jazz applications, the Ger+6 is commonly used as a tritone substitute for the secondary dominant of V. In the key of C, the tritone of V7/V is C-F# on a D7 chord, which is enharmonically spelled as C-G♭ on an A♭ chord.
Progression with N6, It+6, Fr+6, and Ger+6 Chords (PNG and MP3)