The Neapolitan sixth

A chord from outside the key

Every chord at Grade 6 used only the notes of the key (with the raised 7th in a minor key). Grade 7 adds chromatic chords: chords that borrow a note from outside the key for colour, and then lean strongly towards the dominant.

The first is the Neapolitan sixth: the major chord built on the flattened supertonic.

Spelling it in any key

Three steps:

1. Find the supertonic and lower it a semitone: that is the root.
2. Add a major 3rd above it: always the subdominant of the key.
3. Add a perfect 5th above the root: the flattened submediant (in a minor key, the 6th the key already has).

In C major: D flat, F, A flat. In A minor: B flat, D, F. It is labelled N6.

Why a sixth?

The chord is used in first inversion: its 3rd, the subdominant, is in the bass. Figured from that bass, its notes are a 3rd and a 6th above: hence the name.