Neapolitan sixth chord is an altered or modified supertonic chord: flattened root and fifth chord tone (without associating with any relative and parallel keys).
In both major and minor key, the supertonic (ii) chord has harmonic quality of a minor chord. By flattening the root and fifth, the ii is substituted with bII (major chord quality).
Neapolitan sixth chord usually used in a minor key music.
1.b)2. Neapolitan sixth chord in 4-part Writing
The bII is usually in first inversion, written as N6 and progress to dominant (V)chord (may delayed with some diatonic predominant chords: ii, IV, vi and I6-4 and chromatic predominant chords: V/V, vii/V and Augmented Sixth Chords). However, N6 may sometimes be found in root position and may become tonicized chord (vii/N6-N6).
The 3rd tone chord of N6 (bass note) need to be doubled and sometimes, N6 may contains 7th tone.
The chord voicing must have at least one chromatic tone chord resolved and more or equal contrary motionagainst the bass.
The chromatic tone(s) must not be doubled.
Chromatic voice leading must be avoided, except for Leading-tone (LN).