250px|right|thumb|In C Major: A Neapolitan sixth chord in first inversion contains an interval of a sixth between F and Db.

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 &#9837;II<sup>6</sup> or N<sup>6</sup> and normally referred to as a '''Neapolitan sixth chord'''. 

== Origin of the name  ==

The Neapolitan chord is so-called because it is associated with the so-called "Pergolesi, Corelli and Purcell. It was also a favourite idiom among composers in the Classical period, especially 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'').<ref>William Drabkin: 'Neapolitan sixth chord', Grove Music Online ed. L. Macy (Accessed 8 June 2007), <http://www.grovemusic.com></ref> 

It is known as the Neapolitan 'sixth' chord due to the interval of a sixth between the 3rd and the root note of the chord. For example, in the key of C major the Neapolitan sixth chord would consist of the notes D&#9837; (the root note), F (the third) and A&#9837; (the fifth) with the F in the bass to make it a &#9837;II<sup>6</sup> or N<sup>6</sup> rather than a &#9837;II. The interval of a sixth is between F and D&#9837;.

== Harmonic function ==

In dominant, substituting for the IV or ii (particularly ii<sup>6</sup>) chord.  For example, it often precedes an authentic 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 contains the notes - F-A-C.  By lowering the A by a semitone to A&#9837; and raising the C by a semitone to D&#9837;, the Neapolitan sixth chord F-A&#9837;-D&#9837; is formed.  In C minor, the resemblance between the subdominant (F-A&#9837;-C) and the Neapolitan (F-A&#9837;-D&#9837;) 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-A&#9837;-D, and thus lies chromatically between the two primary subdominant function chords.)

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 tonic).

== Further harmonic contexts  ==

A common use of the Neapolitan chord is in 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).

'''ナポリの六度'''、'''ナポリの六の和音'''、あるいは単に'''ナポリ'''とは、西洋音楽において、短2度上（半音上）の音を根音とする長三和音のことである。

この和音は元来、4度上の音（短2度上げ、根音の短6度上に置いたことに由来する。'''ナポリの六度'''の「六」はこの意味である。従って、この短6度音を根音と考える現代の和声学に従う楽曲においても、下属音を最下音に置く第1転回形（六の和音という）の形態として使用されることが多い。

下属和音に準ずる和音として、機能を持ち、特に短調において、IIの代理和音として使用されることが多い。「ナポリの六」はドミナントに進行するのが原則であるため、原則的にトニカに解決されてはならない。

伝統的には'''N'''の記号で表され、また日本で多く使われる和声の教科書である和声 理論と実習では'''-II'''（長調では'''<sup>○</sup>-II'''）の記号で表される。

==各調における構成音==