The three notes that create the distinction between Major (Ionian mode) and Natural Minor (Aeolian mode) scales are 3rd 6th and 7th. One easy way to remember this is that on MAJOR, as the name suggests, we have all the larger intervals. Whereas in minor, those 3 notes get flattened, resulting in shorter intervals, hence called MINOR.
For example, the key of C contains the following notes: C D E F G A B
Flattening the 3rd, 6th and 7th, we get C D Eb F G Ab Bb
However there is a relation between a Major and Minor scales as every Major key has a related minor and vice versa. Looking at it from the perspective of a major key, the 6th degree of the Major key is its related minor. So again going back to the key of C, the 6th degree is A minor, therefore A minor is the related minor of C major. What it means is that both C major and A minor scales share the same identical notes. The difference however is in the voicing as in C major, the tonal centre or root is that C note (C D E F G A B) and in A minor scale the tonal centre or root is the A note (A B C D E F G). If you look at this from the minor key then it is the third degree of the minor key that represents the related major key. In the key of A minor, the third is C, which is the related major of A minor key.
The following two rhymes may help to memorise the differences between Major and minor keys:
- “To make major and minor even, change three six and seven”
- “The chord six, gives major a minor fix”