Neapolitan sixth
theorynee-ah-POL-ih-tan sixth
The first-inversion form of the Neapolitan chord, which is by far its most common voicing
The Neapolitan sixth places the fourth scale degree in the bass (hence the 6 in the name, referring to the first-inversion figured bass), with the root and third of the ♭II chord above. In C minor, this gives F in the bass with D♭ and A♭ above. The bass note F then moves naturally to the dominant G. The chord typically appears in minor keys and functions as a pre-dominant harmony, substituting for ii° or iv. Its distinctive dark colour makes it one of the most expressive chords in tonal music.
Beethoven used the Neapolitan sixth at the climax of the Appassionata Sonata's first movement, creating one of the most dramatic harmonic moments in the piano literature.