The Neapolitan 6th Chord, or N6, is a chromatic alteration of the ii chord, and therefore proceeds to a V chord, either directly, or by passing through other chords first.

Here is an example in C major. Notice that the D and the A have been lowered by a half step.

To improve voice leading, the N6 is usually voiced in first inversion, hence the name "neapolitan sixth", since a triad in first inversion has intervals of a 4th and a 6th.

The N6 can also be used in minor keys. In fact, the N6 sounds better in a minor key, because then only one note of the ii chord needs to be altered.

The use of the flat ii intensifies the harmonic progression: flat ii is a tritone away from V. Therefore, the function of the N6 is to intensify the motion to the dominant.

The Neapolitan Sixth was used extensively by Alessandro Scarlatti, Pergolesi, Paisiello, Cimarosa and later by Beethoven and others.