Major and minor
Major and minor keys that share the same key signature are "relative" to each other. For instance, F major is the relative major of D minor since both have key signatures with one flat. Since the natural minor scale is built on the 6th degree of the major scale, the tonic of the relative minor is a major sixth above the tonic of the major scale. For instance, B minor is the relative minor of D major because the note B is a major sixth above D. As a result, the key signatures of B minor and D major both have two sharps (F and C).