The Neapolitan sixth and augmented sixth chords are the most dramatically powerful chromatic harmonies in the Classical and Romantic vocabulary. Their function is pre-dominant — building tension before the final dominant-tonic resolution.
THE NEAPOLITAN SIXTH (FLAT-II6)
The Neapolitan chord is a major triad built on the flattened second scale degree (bII), typically used in first inversion — hence "Neapolitan sixth." In C minor: the second degree is D; the flattened second is Db; the Neapolitan chord is Db-F-Ab, and in first inversion (F in the bass) it is F-Ab-Db. It appears most often in minor keys but can appear in major keys with the same flatted-second root.
The Neapolitan chord sounds darkly beautiful and has a characteristic harmonic colour that is instantly recognizable in context: a major chord arriving from an unexpected direction, whose lowered second degree (Db in C minor) falls to the leading tone (B) as the chord resolves to V. Its function is pre-dominant: it typically appears before the dominant chord (N6 → V → i), intensifying the approach to the cadence.
Famous uses: the first movement of Beethoven's Piano Sonata No. 14 (Moonlight Sonata, C# minor) features a Neapolitan chord (D major chord in the key of C# minor) as early as bar 3, establishing the brooding, extraordinary character immediately. Schubert uses the Neapolitan constantly in his late works.
AUGMENTED SIXTH CHORDS: THE THREE TYPES
Augmented sixth chords are built around the characteristic interval of the augmented sixth — an interval one half step larger than a major sixth. They appear above the flatted sixth scale degree (Ab in C major/minor) and resolve outward by half step to the octave on the dominant (G-G). The three common types are the Italian sixth (It+6), French sixth (Fr+6), and German sixth (Ger+6), named by tradition after their supposed national origins (Aldwell and Schachter note that these labels have no historical basis and are purely conventional).
All three types share: the interval of an augmented sixth between their outer voices (bVI in the bass to raised-4 in an upper voice), the same resolution (outward to the octave on the dominant), and the same function (pre-dominant, creating maximum harmonic pressure before the dominant). They differ in the note added in the middle: the Italian sixth has only three notes (bVI, 1, #4); the French adds scale degree 2; the German adds scale degree b3 (the most dissonant of the three, and the most commonly used).
HEARING AUGMENTED SIXTH CHORDS
Augmented sixth chords are recognizable by ear through their characteristic resolution: the outer voices expand to an octave (the augmented sixth resolving outward by semitone). The German augmented sixth is the most distinctive — its sound is nearly identical to a dominant seventh chord (the German sixth in C major = Ab-C-Eb-F# sounds enharmonically the same as the dominant seventh in Db: Ab7 = Ab-C-Eb-Gb). This enharmonic equivalence was exploited by Romantic composers for modulation: they would introduce what sounds like a dominant seventh chord, then notate it as a German sixth resolving to a different key entirely.
HISTORICAL EVOLUTION OF CHROMATIC HARMONIES
The Neapolitan and augmented sixth chords developed gradually over the late Renaissance and Baroque periods. Originally, they were specific voice-leading events — particular ways of harmonizing certain melodic gestures — and only later were they systematized as chord types. The Neapolitan first appears as a flattened second scale degree in the soprano voice over a iv6 chord, creating a particularly poignant cadential motion. By Mozart's time, the Neapolitan was understood as an independent chord with characteristic uses.
Augmented sixth chords have a similar history. They originated as chromatic voice-leading: in a phrygian-style cadence, the bass moves from bVI to V by half step, while an upper voice moves from raised 4 to 5 by half step in contrary motion. This creates the augmented sixth interval between bVI and #4. By the late Baroque, this contrapuntal gesture had been theorized into the distinct chord types Italian, French, and German.
WAGNER'S USE OF CHROMATIC HARMONY
Richard Wagner's Tristan und Isolde (1865) revolutionized the use of chromatic harmonies, including augmented sixth chords and Neapolitan-like sonorities. The famous "Tristan chord" (F-B-D#-G#) at the opening of the opera has been analyzed as a half-diminished seventh, an altered ii chord, a French augmented sixth, and other interpretations — the ambiguity is part of its power. Wagner's chromatic technique extended over hours of music, suspending traditional cadential resolution and creating unprecedented harmonic tension that influenced every subsequent composer.
For ear training, Wagner's music is challenging but illuminating. Listening to the opening of Tristan reveals how chromatic harmonies blur the distinction between keys and create a continuous flow of related sonorities without strong cadential punctuation.
LATE ROMANTIC AND IMPRESSIONIST EXTENSIONS
In the late 19th and early 20th centuries, composers extended chromatic harmony beyond Wagner. Cesar Franck's organ and chamber music features dense chromatic textures with frequent enharmonic modulation. Claude Debussy abandoned functional harmony altogether in many passages, using chromatic chords as colour rather than for resolution.
Debussy's Prélude à l'après-midi d'un faune (1894) opens with a chromatic line on solo flute that suggests no clear key. The harmony moves through related chromatic sonorities without traditional cadences, creating the floating, dreamlike quality that defined Impressionist music.
FOCUS PRACTICE FOR CHROMATIC CHORDS
The sound of the Neapolitan in minor key is unmistakable once you know it: a major chord on the lowered 2nd scale degree, in first inversion, leading to the dominant. Listen to the first movement of Beethoven's Piano Sonata Op. 27 No. 2 (Moonlight Sonata) and identify the Neapolitan chord. Then listen to Schubert's "Erlkönig" or his late piano sonatas for further examples.
The Italian sixth has only three notes (bVI, 1, #4). The French sixth adds 2. The German sixth adds b3 (most common, sounds like a dominant seventh). Practice identifying each by playing them on a piano and listening to their characteristic colours. Then find examples in score — Beethoven's piano sonatas contain dozens of augmented sixth chords at structurally important moments.
CHROMATIC CHORDS IN JAZZ
Jazz harmony makes extensive use of chords that function similarly to Neapolitan and augmented sixth chords. The tritone substitution mentioned in earlier lessons — replacing V7 with bII7 — produces a chord that contains the same tritone as V7 but with different bass. The bII7 chord functions as a Neapolitan-like dominant substitute and creates smooth chromatic bass motion (bII7 to I, bass moving down by half step) that is signature of bebop and modern jazz.
The German augmented sixth and the dominant seventh of bII are enharmonically identical (Ab7 = Ab-C-Eb-Gb, and the German +6 in C is Ab-C-Eb-F#). This identity means that jazz musicians frequently use chords that classical theorists would analyze as augmented sixths, simply notating them as dominant sevenths.
SOURCES & FURTHER READING
Aldwell and Schachter. Harmony and Voice Leading. Schirmer/Cengage. Chapter 27: Augmented Sixth Chords. Laitz. The Complete Musician. Oxford University Press. Chapter 22. Open Music Theory — Chromatic Harmonies (openmusictheory.net, CC BY-SA 4.0). Benward and Saker. Music in Theory and Practice, Vol. II. McGraw-Hill. Chapter 14. teoria.com — Advanced Chord Identification.