A minor scale is a different arrangement of steps: whole, half, whole, whole, half, whole, whole. What separates it from major is the third degree, a half step lower — the one note that decides bright or dark.
A minor uses exactly the same keys as C major — every one of them white — and sounds nothing like it. The difference is not which notes but which one is home. Starting on A, the third note is C, three half steps up: a minor third. Starting on C, the third note is E, four half steps up: a major third.
A natural minor: A B C D E F G A, not a black key in sight.
That is what relative keys are. Every major key has a minor that shares its key signature, with its home note a minor third below: C major and A minor, G major and E minor, F major and D minor.
In natural minor the seventh degree is a whole step below home, which does not pull very hard. So in practice composers raise it by a half step, producing the harmonic minor scale — and that is why minor pieces are full of accidentals that are not in the key signature.
A harmonic minor: raise the seventh, G to G♯, and the step home has a pull again.
Common questions
What are the three kinds of minor scale?
Natural minor keeps the plain key signature. Harmonic minor raises the seventh degree a half step; melodic minor raises the sixth and seventh going up and returns to natural minor coming down. In A minor, harmonic adds G♯, and melodic adds F♯ and G♯ on the way up.
What is a relative minor?
It is the minor key that shares a major key’s signature. It starts on the sixth note of the major scale, a minor third below the major key’s home note: A minor for C major, E minor for G major.
Why are there three kinds of minor scale?
Raising the seventh gives the scale a note that pulls toward home, but it leaves a gap of three half steps between the sixth and seventh. Raising the sixth too, on the way up, closes that gap.