What it is
It is the major triad built on the lowered second degree (bII), almost always in first inversion: hence the name “Neapolitan sixth” (N6). It has a predominant function.
- In A minor: Bb/D (Bb D F).
- In C major (borrowed from the minor/Phrygian): Db/F (Db F Ab).
Typical resolution
- In A minor: Bb/D – E7 – Am, or Bb/D – Am/E (cadential 6/4) – E7 – Am.
- In C: Db/F – G7 – C.
Voice leading: the bass (D in A minor) rises to E; the note Bb falls toward G#, possibly passing through A (Bb–A–G#), creating the typical melodic diminished third Bb–G#. In first inversion the bass (the third of the chord) is normally doubled.
Why it sounds this way
The bII has a chromatic relationship with the dominant: the Bb and F of Bb/D lie a semitone from A and E, and the characteristic note (Bb) sits a semitone above the tonic. The result is a painful shadow right before the cadence.
In pop and film
The bII chord in root position is common in Phrygian music, in metal and in dark trap: Am – Bb – Am. It is not a true functional N6 but shares its color.
Real example
The first movement of Beethoven's “Moonlight” Sonata (Op. 27 No. 2, 1801), in C# minor, introduces almost immediately a D major chord in first inversion: it is the Neapolitan that colors the opening.