DontRunMeOver

A neapolitan sixth is a major chord whose root note is one semitone above the root note of the tonic chord.

In simpler terms, if you're playing in E major, the neapolitan sixth is the F major chord. If you're playing in E minor the neapolitan sixth is also the F major chord. If you're in B major, the NS is C major. In B minor its C major and so on.

Its called a 'sixth' because the neapolitan chord (the F major chord is the first example) is played in its first inversion. That means that instead of the bass note being an F, you use A as the bass note... the 6th semitone in the chord.

Overall, its meant to give a 'spooky/ghostly' feel to the piece, this is what is was mostly used for in classical music. Its used regularly in jazz, funk and latin music though, so the only consistent effect it really has is to make the music sound like it 'drifted' upwards and then will sound like it has been 'lowered' back down when it returns to the tonic (which it usually does).

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