There are areas in a song that can change keys many times without changing the song's key signature. They are called key areas and are explained in the attached PDF :

https://www.dropbox.com/s/fr3y5ungopeuzdd/Key%20areas.pdf?dl=0

This is how those out of the song key signature chords are explained. Basically any chord can come after any other chord, even if the modulation is abrupt.


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