Even though the key signature for a major and its relative minor are the same, that is only part of the story.
When you see that A Minor has the same key signature as C Major, what you are seeing is the "natural minor" scale. This is simply the notes implied by the key signature, and so is identical (but down a third) from the relative major. If you play purely in the natural minor, you are really closer to the Aeolian mode than a minor key.
There is also the "harmonic minor" and "melodic minor" which, in common practice period, are much more commonly used than natural minor. Harmonic minor has a raised seventh. This makes the dominant (V) triad a major triad which increases its need to resolve to the tonic (I). It is used for harmonies (obviously based on the name) for this reason, but it makes the step from 6 to 7 an augmented second (i.e. minor third) so its isn't used melodically very often. Which is why there is also a Melodic Minor.
Melodic minor is tricky because it varies depending on whether the melody is ascending or descending. Descending is easy, it is identical to natural minor. Ascending is similar to the harmonic minor with its raised seventh (creating what is known as a leading tone, i.e. a half step below the tonic that wants to resolve up to the tonic.) But the ascending scale also has a raised sixth, which eliminates the augmented second between 6 and 7.
Thank you for this. I learned to write natural, harmonic, and melodic minor scales when I was a kid, but I never knew this explanation. It makes perfect sense!
When you see that A Minor has the same key signature as C Major, what you are seeing is the "natural minor" scale. This is simply the notes implied by the key signature, and so is identical (but down a third) from the relative major. If you play purely in the natural minor, you are really closer to the Aeolian mode than a minor key.
There is also the "harmonic minor" and "melodic minor" which, in common practice period, are much more commonly used than natural minor. Harmonic minor has a raised seventh. This makes the dominant (V) triad a major triad which increases its need to resolve to the tonic (I). It is used for harmonies (obviously based on the name) for this reason, but it makes the step from 6 to 7 an augmented second (i.e. minor third) so its isn't used melodically very often. Which is why there is also a Melodic Minor.
Melodic minor is tricky because it varies depending on whether the melody is ascending or descending. Descending is easy, it is identical to natural minor. Ascending is similar to the harmonic minor with its raised seventh (creating what is known as a leading tone, i.e. a half step below the tonic that wants to resolve up to the tonic.) But the ascending scale also has a raised sixth, which eliminates the augmented second between 6 and 7.