Modes apply to scales, depending on what mode you play in depends on what notes you flat (or in one of the modes you sharp the 4th I believe, forget what it's called though), like in one of them you flat the 3rd, which means if you were to take a scale and write out all the notes starting with the letter of the scale you are playing you would go to the 3rd note on the list and flat it (That means you move it one note down, but you don't skip the sharps).

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