Reference

Neapolitan Sixth

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

When using a Neapolitan sixth in major keys, its fifth should be lowered in order to create the same chord accidentals as in the minor key:

Neapolitan Sixth, C Major

To learn more about Neapolitan sixth chords click here.