Major and Minor 3rds, 6ths and 7ths

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When 3rds, 6ths and 7ths are formed by playing the third, sixth or seventh note of a Major scale with the first note or “tonic”, the intervals formed are Major 3rds, Major 6ths and Major 7ths.
Lowering the upper note of each of these intervals  by one 1/2 step (by one key) changes the quality of the interval from Major to minor.
Note to more advanced students: lowering the 3rd, 6th and 7th of a Major scale does two very important things:
  1. It produces a “natural minor scale”.
  2. It tells us what the key signature of the “relative” or “tonic” minor will be. For instance, C Major, which has no sharps or flats in the key signature, will switch to three flats (Bb, Eb, Ab) for C minor.

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