Neapolitan sixth

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. The Neapolitan chord is so-called because it is associated with the so-called "Neapolitan school", which included Alessandro Scarlatti, Pergolesi, Paisiello, Cimarosa and other important 18th-century composers of Italian opera; but it seems to have been an established if infrequent harmonic practice by the end of the 17th century, being used by Carissimi, Corelli and Purcell. It was also a favourite idiom among composers in the Classical period, especially Beethoven, who extended its use to root-position and second-inversion chords (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). 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 (the root note), F (the third) and ♭A (the fifth) with the F in the bass to make it a ♭II6 or N6 rather than a ♭II. The interval of a sixth is between F and D♭.
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