A Neapolitan chord is simply a major triad built on the lowered second degree of a major or minor scale
Makes the 2nd chord major by flatting the 2nd degree of the scale
Neapolitan generally 1st inversion
Neapolitan called the tri-tone substitution
For instance go from vi to N to I
Usually used as a predominant
Borrowed Chord - in major key, want to use diminished 2 chord, can take it from the minor, take the parallel chord quality, for instance instead of FAC have F Ab C, go from the major chord to the minor chord, still in the progression, just changes the quality

Augmented 6th Chords:
3 kinds, Italian, French, and German
Bottom note of the augmented 6th chord is the flatted 6th of the scale, same sound as dominant 7th chord, Ab C Eb F#, Ab to F# is an augmented 6th
Build a major triad with a dominant seventh, but is a augmented 6th, Ab resolve out to a G, predominant chord
German - German has everything, adds the fifth
Italian - are missing one of the notes,
French - Have an augmented 4th

An altered tone is always present
a chord represents a chromatic modification to the 4 chord