Circle of fifths
Every chord you could play on each degree of a key, ranked by how often these songs actually play it — the ones in the key, and the ones worth reaching outside for.
1
2
3
4
5
6
7
Where these songs differ from the books
The Neapolitan is standardly written in first inversion — that is why it is called the Neapolitan sixth — with its third doubled. These songs play it in root position, and never invert it. Open Music Theory, Neapolitan Sixth Chords