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5.5.5 Related Keys: Notes in a Key (Examples)
The notes in a key determine how closely related the relationship is. The more notes two keys have in common, the more closely the keys are related. Coziness of relationship between keys plays a big role in modulation. Keys that share the identical set of notes have the closest relationship—the majors and their relative minors.
For example, the key of C major and A (natural) minor use exactly the same seven notes. The two keys are simply organized around two different tonal centres.
Equally important are keys that have all but one note in common—six out of seven notes. For example, the key of C major has these seven notes:
C D E F G A B
The key of G major has these seven notes:
G A B C D E F♯
Six out of seven notes belong to both keys. So the keys of C major and G major are closely related.
Similarly, the key of F major has these seven notes:
F G A B♭ C D E
Six out of seven notes belong to both the key of C major and the key of F major. So the keys of C major and F major also have a cozy relationship.