One of the most commonly-used chromatic chords in the Classical period (and beyond) is the Neapolitan 6th chord, or “N6”.
This is a major chord, built from the flattened supertonic note. It is found in both major and minor keys (but is more common in minor keys).
In C major the supertonic note is D. Flatten the supertonic = Db. Build a major triad = Db major.
In A minor the supertonic note is B. Flatten the supertonic = Bb. Build a major triad = Bb major.
The Neapolitan 6th chord is often used in first inversion (which is why it’s called a “6th” chord – after the figured bass symbol for first inversion chords). It can be found in other inversions too, however.
Usually, it is followed by a dominant chord, or a chord which behaves like a dominant (such as Ic, or vii°, etc.)
Here is an example of a Neapolitan 6th chord in Mozart’s Piano Sonata in A minor (K310). N6 is Bb major, which moves to a chord of vii°7.
