I was under the impression it had to do with each note can only be used once. It sounds a bit strange. We have 7 notes A B C D E F & G. Each can only be used once therefore if a scale had Bb in it the scale cannot also have a B so one might use Cb. Similarly if a scale had an F# it cannot also have an F so E# could be used.

I can’t remember if it was Guy Mitchelmore or Gracie Terzian that had a whole video dedicated to when and where this occurs.

My thoughts
Tony

Last edited by Teunis; 12/26/22 02:56 PM.
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