Sonata form
In the exposition we hear all the main material: the first tune- or group of tunes - in the main key, then a contrasting tune or tunes in a related key (normally the “dominant” i.e. the key on the 5th note of the scale of the main key, or the relative minor). Or in the case of a first section in a minor key, one might frequently hear the second subject or subjects in the relative major.