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In music theory, a Neapolitan chord (or simply a "Neapolitan") is a major chord built on the lowered second (supertonic) scale degree. It usually appears in the first inversion, conforming to the traditional classification of a sixth chord and notated in figured bass symbols as bII6 or N6. Thus it is commonly known by the name Neapolitan sixth. The Neapolitan chord is so-called because it is erroneously attributed to Baroque composers in Naples, Italy, who used it at dramatic moments in their operas. Although the Neapolitan chord was used by these composers, it originated earlier. In tonal harmony, the function of a Neapolitan chord is to prepare the dominant, substituting for the IV or ii (particularly ii6) chord. For example, it often precedes a perfect cadence, where it functions as a subdominant (IV). In such circumstances, the Neapolitan sixth may be considered to be a type of chromatic alteration of the subdominant. For example, in C major, the IV (subdominant) triad in root position is spelled F-A-C. By lowering the A by a semitone to Ab and raising the C by a semitone to Db, the Neapolitan sixth chord F-Ab-Db is formed. In C minor, the resemblance between the subdominant (F-Ab-C) and the Neapolitan (F-Ab-Db) is even stronger, since only one note differs by a half-step. (Note that the Neapolitan is also only a half-step away from the diminished supertonic triad in minor in first inversion, F-Ab-D, and thus lies chromatically between the two primary subdominant function chords.) 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 Db generally moves down by step to the leading tone B-natural (creating an awkward 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 (Ab) 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. If a I6/4 (or i6/4) chord is placed between the Neapolitan sixth and the dominant, 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 top part. If the root or (doubled) third is in the top part, 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 lowest part. (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 Neapolitan chord is sometimes used in root position (in which case there may be even more concessions regarding consecutive fifths, similar to those just discussed); an example occurs in the second 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". 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 happens to also be the tonic). The final common use of the Neapolitan chord is in tonicizations and modulations to remote keys. Occasionally, a minor seventh is added to the Neapolitan chord, which creates a potential secondary dominant that can allow tonicization or modulation to the bV/
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