Relative and parallel keys

Every major key has two closely related minor keys, and they are related in completely different ways.

Relative keys

C major and A minor use exactly the same notes and the same key signature. They differ only in which note behaves as home.

To find the relative minor: count down a minor third from the major tonic — three semitones — or up a sixth, which lands in the same place.

Because relatives share a signature, moving between them requires no accidentals at all. A piece can slip from C major to A minor and back without a single written change — which is why it is one of the most common moves in music.

Parallel keys

C major and C minor share the note C as tonic. They do not share notes: C major has no accidentals, C minor has three flats. Three scale degrees differ — the third, sixth and seventh.

To find the parallel minor: keep the tonic, flatten degrees 3, 6 and 7.

Moving between parallels is dramatic rather than seamless. The tonic stays put while the colour around it changes completely — a major chorus after a minor verse on the same root is a familiar and very effective device.

Which one to reach for

Relative for continuity. The notes do not change, so the shift is smooth and often barely noticed. Verse in A minor, chorus in C major is a standard pop move.

Parallel for contrast. The tonic anchors while everything else shifts. Also the source of borrowed chords — using a chord from the parallel key inside the current one, like the minor iv chord appearing in a major key.