Relative key
Major/minor relative with the same key signature.
What it means
Two keys are called relative when they have the same key signature and are of different modes. The relative minor lies a minor third below the major tonic: A minor to C major, E minor to G major, G minor to B♭ major. Both keys draw from the same stock of notes; only which note acts as the centre differs.
Musical use
Because the key signature says nothing about which of the two is meant, the music must show it: through the closing note, through emphasis on the tonic, and above all through the leading tone, which in minor appears as an accidental within the bar. A piece with two sharps is recognizable as B minor only once the A♯ appears.
The change to the relative key is the simplest modulation of all, because no accidental needs to change; it suffices to shift the centre. This is why it is, in many forms, the first goal of a tonicization, especially in minor pieces, whose middle sections often stand in the relative major. In function theory the relative sonority is designated specifically and counts as a representative of the main function to which it belongs.
The relative key is to be distinguished from the parallel key (tonic major/minor), which has the same tonic with a different mode — C major and C minor. There the tonic stays and the key signature changes by three signs; here the key signature stays and the tonic changes. In English usage the terms are swapped, which leads to misunderstandings in translation.
Relative keys have the same key signature; their tonics lie a minor third apart: C major and A minor, G major and E minor. In English the relative key is called relative key. Modulation into the relative key is common in major-minor music. In the circle of fifths, relative keys are usually shown in the inner circle next to the corresponding major key.
Performance practice
Determine for every piece whether major or minor is meant before you begin; the key signature alone does not suffice. Watch the shift into the relative key — it changes the colouring without a new key signature appearing, and is easily missed when playing.