The Neapolitan sixth chord is built on the second degree of the major and minor scales. Generally, it is used in first inversion. Its root is lowered. It is commonly used to reach the dominant chord or the tonic chord in second inversion when performing a cadence:
|Neapolitan Sixth, A Minor
|Neapolitan Sixth, C Major
To learn more about Neapolitan sixth chords click here.