In Classical music theory, a Neapolitan chord (or simply a "Neapolitan") is a major chord built on the lowered (flattened) second (supertonic) scale degree. In Schenkerian analysis, it is known as a Phrygian II,[1] 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",[2] some analysts prefer the latter because it indicates the relation of this chord to the supertonic.[3] 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.[3] Therefore, like the augmented sixth chords it should be assigned to a separate category of chromatic alteration.
In European Classical music, 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.[4] In B major or B minor, for example, a Neapolitan sixth chord in first inversion contains an interval of a minor sixth between E and C.
The Neapolitan sixth chord is an idiom specific to classical music. Other music traditions often feature ♭II harmonies (ex. C major chord in the keys of B major or B minor), but usually in root position. These are sometimes referred to as "Neapolitan" chords, but these rarely follow the classical voice-leading and chord functions described below. For examples and discussion, see Tritone substitution, or the section "In popular music" below.