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#.




Lower it to F natural.




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.




Lower it to Bbb.




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.