A Neapolitan sixth chord is a chord on ④ with a minor third and a minor sixth. It typically occurs in minor, although you may occasionally encounter it in major as well. In both modes, the sixth doesn’t belong to the scale but is a chromatically lowered version of ➋, giving its specific flavour to this chord. In major, the third is also chromatically lowered, that is, ♭➏. The Neapolitan sixth chord is often followed by a (major) triad or seventh chord on ⑤ with a ♭➋–➐ snippet in the melody, creating an expressive melodic diminished third that may or may not be filled in with a passing note.
In a three- or four-part setting, the cross-relation between the minor sixth of the Neapolitan sixth chord and the fifth of the triad on ⑤ is admitted.
