The Neapolitan 6th is a chromatic chord built on the flattened supertonic of a key.
It is predominantly written in its 1st inversion and has a very distinctive sound.
For example, in the key of C major and C minor the Neapolitan sixth would be a D flat chord in its 1st inversion:
The Neapolitan sixth is usually found in minor keys.
Here is a typical chord progression that uses a Neapolitan 6th to move towards chord V and then returning to chord I.
Neapolitan 6th Example
There is some debate as to the origin of the name Neapolitan Sixth.
It can be found in 17th century music and so pre-dates the “Neapolitan School”.
However, it is thought that its popularity with 17th and 18th century composers in Naples did have some influence on its name.
Here is an example of a Neapolitan sixth chord taken from Beethoven’s Piano Sonata, Op. 31 No.2:
You can hear the distinctively dramatic and rich sound that the unexpected chord produces. It feels like it is trying to “pull” the music to a new tonality.
Modulating Using The Neapolitan Sixth
The Neapolitan sixth is a very effective means of modulating down a semitone.
The tonic in the existing key becomes the flattened supertonic in the new key.
In this example below, we are in the key of B flat major and I am looking to modulate down a semitone to A minor.
I have used a B flat major chord in its 1st inversion (Ib) at the beginning of the 2nd bar.
This corresponds to the flattened supertonic of A minor and so effectively becomes a Neapolitan Sixth in A minor.
This allows me to use the typical progression to the tonic of the new key via a perfect cadence (V-I):