Modulation is a “journey” into a new emotional space. When a composer moves music from C major to A minor, the listener feels the change of light even without naming its cause.
There are various types: diatonic (through common chords), chromatic (through altered pitches), enharmonic (by reinterpreting a chord in a new context). The Classical sonata form is built entirely on modulations — the primary theme in one key, the secondary in another, with the recapitulation uniting them. Schubert and Liszt discovered elliptic modulation — a sudden leap to a distant key that shocks and enchants at once.
