A Neapolitan chord is simply a major triad that is built on a special note.
This note is the lowered second degree (the supertonic) of a major or minor scale.
Let's build the Neapolitan of C Minor.
First, figure out the second scale degree. In C Minor, this is D.
Next, we will lower it to Db.
Now build a major triad. This results in Db – F – Ab.
A Db major chord is the Neapolitan of C Minor.
Next, let's build the Neapolitan of E Minor.
Again, we need to figure out the second scale degree. In E Minor, this is F#.
Now, build a major triad. This results in F – A – C.
Thus, an F major chord is the Neapolitan of E Minor.
Next, let's build the Neapolitan of Ab Major.
In this key, the second scale degree is Bb.
Now, build a major triad. This results in Bbb – Db – Fb.
Thus, a Bbb major chord is the Neapolitan of Ab Major.
For our final example, let's build the Neapolitan of A Major.
In this key, the second scale degree is B.
Now, build a major triad. This results in Bb – D – F.
Thus, a Bb major chord is the Neapolitan of A Major.