Relative keys
Relative keys are a major key and a minor key that share the same key signature and the same keys on the piano, but have different tonics. C major and A minor are relative keys: neither has any sharps or flats.
To find the relative minor, count three scale notes down from the major tonic, the tonic included: C, B, A. A is the sixth degree of C major, a minor third (three half steps) below C. The other keys work the same way: G major (one sharp, F♯) and E minor; F major (one flat, B♭) and D minor.
The key signature is the same, but the mood differs: C major sounds bright, A minor darker, because its scale starts on another note and the half steps fall in other places. The tonic – the note and chord a piece comes home to – tells you which of the two keys you are in.
Mind the terms in other languages. Lithuanian “paralelinės tonacijos” and Russian «параллельные тональности» mean relative keys. The English term parallel keys means something else: a major and a minor key with the same tonic, such as C major and C minor.
What it sounds like
The same tune in the relative key sounds as if lit differently: bright in major, darker and sadder in minor – and yet the notes are the same white keys.
Where it is taught: the studio’s interactive textbook, chapter 8, “Minor: A minor and relative keys”.