The "Neapolitan Sixth" is a type of chord in music theory that involves an altered harmony typically used within the context of common-practice harmony, especially during the Classical and Romantic periods. The Neapolitan Sixth chord is known for its distinctive sound and its tendency to create harmonic tension and resolution.
The Neapolitan Sixth chord is built on the lowered second degree of the scale (in minor keys) or the lowered sixth degree (in major keys). It consists of a root, a minor third above the root, and a diminished fifth above the root. This creates a unique and somewhat dissonant sound that seeks resolution.
The Neapolitan Sixth chord often resolves to the dominant chord or the tonic chord. In its resolution, the diminished fifth of the Neapolitan Sixth chord typically moves outward by a half step to the third of the dominant chord, while the root moves downward by a half step to the fifth of the dominant chord.