The Neapolitan-Sixth: A "Chromaticized" II Chord

What is it?

In minor mode, a II chord in first inversion sometimes appears NOT with diatonic scale degree 2 in the melody, BUT WITH chromaticized (lowered) scale degree 2 ("flat" 2 in some keys (c-minor, g-minor, etc.) "natural" 2 in keys like c# minor, f# minor, etc.). In figured bass notation, the result is a lowered sixth above the bass or "Neapolitan" sixth.

In root position, the Neapolitan chord is a major triad whose root is lowered scale degree 2 ("flat" 2 or "nat." 2) Because of its distinctive position a semitone above the tonic, the Neapolitan chord is sometimes called Phrygian II.

Closer look at Example 1

Note that in minor mode, the third and fifth of the Neapolitan are diatonic, so that only a single accidental is necessary. However, only the third of the chord is diatonic in major mode, so that TWO accidentals are necessary; both root and fifth must be lowered by a semintone.

Primary Function (PD)

The Neapolitan sixth chord most frequently appears in first inversion as part of an intensified cadential progression. In this respect, its function is much like diatonic II⁶. The melodic direction of lowered scale degree 2 is predetermined--it must move down, and it must move to the dominant. Leading bII⁶ directly to I does not fully resolve the chromatic element. When moving to V⁽⁷⁾, lowered scale degree 2 is "corrected" in another voice.

Closer look at Example 2

Notice that when moving directly to V, the melodic progression of a diminished third will normally result--this melodic succession is the "calling card" of Phrygian II.

Can you hear the Neapolitan Sixth Chord in the following musical excerpts?

Beethoven Bagatelle Op. 119, No. 9. Chopin Nocturne Op. 48, No. 1.

Voice-leading Tips

Oftentimes, the melodic progression of a diminished third is filled in with a passing tone (scale degree 1) in conjunction with the V . Another possibility is switching to V.
Closer look at Example 3
Note that the least problematic voice to doubling is the bass of the Neapolitan II⁶ because contrary motion is immanent. In this case, the bass moves up to V and the remaining voices move down. Other doublings are possible--when doubling lowered scale degree 2 (rare), keep the resulting chromatic succession direct (i.e., in the same voice, preferably an inner voice). This happens in the Beethoven Excerpt above.
bII⁶ can occur in non-cadential situations as well. Some possibilities include moving to a V⁴₂ or VII^o4₃.
bII in root position occurs less frequently because of the progression of a tritone in the bass, but it can lead convincingly to chords built on the leading tone.

Non-cadential and Other Exotic Uses of the Neapolitan

The Neapolitan may be the agent of modulation, as in the Chopin Prelude in C-minor, where a root position bII functioned as IV in A-flat major (VI).

Hear Chopin: Prelude in C-minor
Phrygian II may also be tonicized, as in the opening of the Beethoven Sonata in F-minor, Op. 57, where a contapuntal cadence emphasizes the striking appearance of bII.

Hear Beethoven: Sonata in F-minor, Op. 57
The Neapolitan six-four is quite rare--in fact, I have never seen one. It can usually be explained using contrapuntal apparatus (i.e., as a neighbor to V or bVI).