The Neapolitan sixth chord is a first inversion Major chord built on the flat supertonic. The sixth refers to the standard inversion symbol for first inversion. The flat supertonic is the second note of the scale lowered by one half step. For example in the key of C Major, the flat supertonic note would be Db. The N6 chord in the key of C is the first inversion of the Db Major chord.
