The relative key: for a plain major or minor key, the same key signature read
in the other mode.
C major and A minor share a signature, and so are each other's relative; the
relation is its own inverse for a plain major or minor key. Because the new
tonic is read off the circle of fifths rather than transposed by semitones,
the relative of Db major is Bb minor and not its enharmonic A# minor.
A key that is not a plain major or minor is read through the mode its third
names and stepped from where its tonic stands on the circle, the origin every
relation here shares: the relative of D dorian is F major, the relative of the
D minor its third names. The shared-signature reading does not carry over to
those — D dorian is written with no accidentals and F major with one — and the
modes have no relative of their own for it to be measured against.
The relative key: for a plain major or minor key, the same key signature read in the other mode.
C major and A minor share a signature, and so are each other's relative; the relation is its own inverse for a plain major or minor key. Because the new tonic is read off the circle of fifths rather than transposed by semitones, the relative of Db major is Bb minor and not its enharmonic A# minor.
A key that is not a plain major or minor is read through the mode its third names and stepped from where its tonic stands on the circle, the origin every relation here shares: the relative of D dorian is F major, the relative of the D minor its third names. The shared-signature reading does not carry over to those — D dorian is written with no accidentals and F major with one — and the modes have no relative of their own for it to be measured against.