The distance from the tonic of a major key to the tonic of its relative minor
is always down a minor third; for example, C down to A, G down to E,
and F down to D. To help remember this relationship, think of minor as
feeling "down", then count down three scale degrees from the tonic of the
major scale. Notice also that the tonic of the relative minor is the same
note as the sixth degree of the relative major. Using this knowledge, it is
possible to find the key signatures of minor keys without actually writing
the scales out.
Another way to find minor key signatures quickly is by counting up a
minor third from the tonic of the minor key to the tonic of its relative
major; for example, from A up to C, E up to G, and D up to F. Think of
major as being an "up" feeling, then count up three scale degrees from the
tonic of the minor key. Notice that the tonic of the relative major key is the
same note as the third degree of the minor key. Once you find the tonic of
the relative major, you can then name the key signature for both major and
relative minor.
Parallel Minor Keys
There is another relationship between major and minor keys that is also
useful in comparing scale structure and key signatures. This is the
relationship between parallel major and minor keys. Parallel keys are
those that are built on the same tonic but have different key signatures, for
example, C major and C minor. The parallel relationship offers a step by
step comparison of the construction of the two scales, and the result is a
simple and very common way to describe the natural minor scale: as a
major scale with a lowered third, sixth, and seventh. This view is accurate
and provides a handy way to quickly compare the scales. However, this
comparison is only technical. The different emotional quality of natural
minor scales results from a structure that is very different from major
scales, and to the listener, minor scales are in no way heard as "altered
major scales". As a writer or an improviser, musicians must learn to hear
and use the different scale qualities to create melodies with the subtle
emotional effects that give music its unique power of expression.