Neapolitan sixth
In music theory, a Neapolitan chord (or simply a "Neapolitan") is a major chord built on the lowered second (supertonic) scale degree. Also, in Schenkerian analysis, it is known as Phrygian II, since in minor scales the chord is built on the notes of the corresponding Phrygian mode.
Although it is sometimes indicated by an "N" rather than a "♭II", some analysts prefer the latter because it indicates the relation of this chord to the supertonic. The Neapolitan chord does not fall into the categories of mixture or tonicization. Moreover, even Schenkerians like Carl Schachter do not consider this chord as a sign for a shift to the Phrygian mode. Therefore, like the augmented sixth chords it should be assigned to a separate category of chromatic alteration.
The Neapolitan 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, for example, a Neapolitan sixth chord in first inversion contains an interval of a minor sixth between F and D♭.
Especially in its most common occurrence (as a triad in first inversion), the chord is known as the Neapolitan sixth:
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.
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