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:
- It produces a “natural minor scale”.
- 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.