Mandolin Theory – How to Build Neapolitan Chords, Part I

In music theory, a Neapolitan chord (or simply a "Neapolitan") is a major chord built on the lowered second (supertonic) scale degree. A Neapolitan chord 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. [...]