The Neapolitan Sixth is the first inversion of a major chord on the flatten supertonic, the second degree of the major and minor scales used. In theory it is commonly used to intensify the approach to a dominant chord because of the tri-tone interval. Another is to reach the tonic chord in second inversion when performing a cadence.
In the key of C the flattened supertonic is D flat, the major chord would be D flat, F and A flat. Conventionally, the triad is in the first inversion, hence the 'sixth.' Therefore, the bass note of the D flat major triad is F. Another theory is it is occasionally used in root position to modulate down a semitone.
One last thing. In a major key the 5th of a Neapolitan sixth is often times lowered to keep the same chord accidentals that would be in a minor key.
Then of course there is the Neapolitan Scale.
[Edited by noticingthemistake on 10-09-2003 at 12:55 PM]
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