Every major scale has a relative minor scale. The relative minor scale starts in the sixth note or degree of the major scale. Relative scales have the same notes. Here are the C major and its relative minor scale, the A minor scale:
A minor is the relative minor scale of C major and C major is the relative major scale of A minor. Relative major scales start in the III degree of minor scales.
Relative scales share the same key signature. Some examples:
See: Names of degrees