In music theory, a Neapolitan chord (or simply a "Neapolitan") is a major chord built on the lowered second (supertonic) scale degree. It most commonly occurs in first inversion so that it is notated either as ♭II6 or N6 and normally referred to as a Neapolitan sixth chord. In C, a Neapolitan sixth chord in first inversion contains an interval of a sixth between F and D♭. In Schenkerian analysis, it is known as Phrygian II.
In doing roman numeral analysis, it is sometimes indicated by an "N" rather than a "♭II".
 Origin of the name
Especially in its most common occurrence (as a triad in first inversion), the chord is known as the "Neapolitan sixth":
- The interval between the bass note and the root of the chord is a minor sixth. For example, in the key of C major or C minor the chord consists of D♭ (the root note), F (the third of the triad), and A♭ (the fifth of the triad) – with the F in the bass, to make it a ♭II6 or N6 rather than a root-position ♭II. The interval of a minor sixth is between F and D♭.
- The chord is called "Neapolitan" because it is associated with the Neapolitan School, which included Alessandro Scarlatti, Pergolesi, Paisiello, Cimarosa, and other important 18th-century composers of Italian opera; but it seems already to have been an established if infrequent harmonic practice by the end of the 17th century, used by Carissimi, Corelli, and Purcell. It was also a favorite idiom among composers in the Classical period, especially Beethoven, who extended its use in root-position and second-inversion chords also (examples include the opening of the String Quartet op. 95, the second movement of the Hammerklavier Sonata, and near the beginning and again in the recapitulation of the first movement of the Moonlight Sonata).
 Harmonic function
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In tonal harmony, the function of the Neapolitan chord is to prepare the dominant, substituting for the IV or ii (particularly ii6) chord. For example, it often precedes an authentic cadence, where it functions as a subdominant (IV). In such circumstances, the Neapolitan sixth is a chromatic alteration of the subdominant, and it has an immediately recognizable and poignant sound. For example, in C major, the IV (subdominant) triad in root position contains the notes F, A, and C. By lowering the A by a semitone to A♭ and raising the C by a semitone to D♭, the Neapolitan sixth chord F-A♭-D♭ is formed. In C minor, the resemblance between the subdominant (F-A♭-C) and the Neapolitan (F-A♭-D♭) is even stronger, since only one note differs by a half-step. (The Neapolitan is also only a half-step away from the diminished supertonic triad in minor in first inversion, F-A♭-D, and thus lies chromatically between the two primary subdominant function chords.)
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The Neapolitan sixth chord is particularly common in minor keys. As a simple alteration of the subdominant triad (iv) of the minor mode, it provides contrast as a major chord compared to the minor subdominant or the diminished supertonic triad. The most common variation on the Neapolitan chord is the Neapolitan major seventh, which adds a major seventh to the chord (this also happens to be the tonic).
 Further harmonic contexts
A common use of the Neapolitan chord is in tonicizations and modulations to different keys. It is the most common means of modulating down a semitone, which is usually done by using the I chord in a major key as a Neapolitan chord (or a flattened major supertonic chord in the new key, a semitone below the original).
Occasionally, a minor 7th or augmented 6th is added to the Neapolitan chord, which turns it into a potential secondary dominant that can allow tonicization or modulation to the ♭V/♯IV key area relative to the primary tonic. Whether the added note were notated as a minor 7th or augmented 6th largely depends on how the chord resolves. For example, in C major or C minor, the Neapolitan chord with an augmented 6th (B-natural added to D♭ major chord) very likely resolves in C major or minor, or possibly into some other closely related key such as F minor. However, if the extra note is considered an added seventh (C♭), this is the best notation if the music is to lead into G♭ major or minor. (If the composer chose to lead into F♯ major or minor, very likely the Neapolitan chord is notated enharmonically based on C♯ (for example: C♯-E♯-G♯-B), although composers vary in their practice on such enharmonic niceties.)
Another such use of the Neapolitan is along with the German augmented sixth chord, which can serve as a pivot chord to tonicize the Neapolitan as a tonic. Play (help·info) In C major/minor, the German augmented sixth chord is an enharmonic A♭7 chord, which could lead as a secondary dominant to D♭, the Neapolitan key area. As the dominant to ♭II, the A♭7 chord can then be respelled as a German augmented sixth, resolving back to the home key of C major/minor.
In rock and popular music, examples of its use, notated as N and without "traditional functional connotations," include Fleetwood Mac's "Save Me", Journey's "Who's Crying Now", and The Rolling Stones' "Mother's Little Helper". The Neapolitan sixth also figures prominently, with its traditional function, in the main title music by Harry Gregson-Williams and John Powell for the 1998 movie Antz. For more examples of Neapolitan Chords in popular music, see List of songs with chromatic harmony.
 Voice leading
Because of its close relationship to the subdominant, the Neapolitan sixth resolves to the dominant using similar voice-leading. In the present example of a C major/minor tonic, the D♭ generally moves down by step to the leading tone B-natural (creating the expressive melodic interval of a diminished third, one of the few places this interval is accepted in traditional voice-leading), while the F in the bass moves up by step to the dominant root G. The fifth of the chord (A♭) usually resolves down a semitone to G as well. In four-part harmony, the bass note F is generally doubled, and this doubled F either resolves down to D or remains as the seventh F of the G-major dominant seventh chord. In summary, the conventional resolution is for all upper voices to move down against a rising bass.
Care must be taken to avoid consecutive fifths when moving from the Neapolitan to the cadential 6
4. The simplest solution is to avoid placing the fifth of the chord in the soprano voice. If the root or (doubled) third is in the soprano voice, all upper parts simply resolve down by step while the bass rises. According to some theorists, however, such an unusual consecutive fifth (with both parts descending a semitone) is allowable in chromatic harmony, so long as it does not involve the bass voice. (The same allowance is often made more explicitly for the German augmented sixth, except in that case it may involve the bass – or must, if the chord is in its usual root position.)
The flattened major supertonic chord is sometimes used in root position (in which case there may be even more concessions regarding consecutive fifths, similar to those just discussed). The use of a root position Neapolitan chord may be appealing to composers who wish for the chord to resolve outwards to the dominant in first inversion; the flattened supertonic moves to the leading tone and the flattened submediant may move down to the dominant or up to the leading tone. Although the flattened submediant moves similarly to the flattened supertonic, the transition from a root position Neapolitan chord to a first inversion dominant has more outward motion than the first inversion, whose notes all move towards tonic. An example of a flattened major supertonic chord occurs in the second to last bar of Chopin's Prelude in C minor, Op. 28, No. 20. In very rare cases, the chord occurs in second inversion; for example, in Handel's Messiah, in the aria Rejoice greatly.
 See also
- Jonas, Oswald(1982). Introduction to the Theory of Heinrich Schenker (1934: Das Wesen des musikalischen Kunstwerks: Eine Einführung in Die Lehre Heinrich Schenkers), p.29n29. Trans. John Rothgeb. ISBN 0-582-28227-6.
- William Drabkin: 'Neapolitan sixth chord', Grove Music Online ed. L. Macy (Accessed 8 June 2007).
- Stephenson, Ken (2002). What to Listen for in Rock: A Stylistic Analysis, p.90. ISBN 978-0-300-09239-4.
- Christ, William (1973), Materials and Structure of Music 2 (2 ed.), Englewood Cliffs, NJ: Prentice Hall, pp. 146–7, ISBN 0-13-560342-0, OCLC 257117 LCC MT6 M347 1972