How do keys create contrast in a sonata-form exposition?

In a sonata-form exposition, contrast is primarily achieved through the opposition of tonalities, or key areas, rather than through thematic differentiation alone.1

The exposition typically begins firmly rooted in the tonic, or home, key of the work. As the section progresses, the music modulates—or changes key—to a closely related but distinct key. This process is essential for establishing a sense of dynamic contrast and tension that the subsequent development and recapitulation sections will later strive to reconcile.123

The choice of the second key is generally predictable based on the home key:
* If the home key is major, the dominant key is typically chosen.1
* If the home key is minor, the relative major is usually selected.12

While composers often introduce different themes to help the listener identify these tonal shifts, the structural significance of the key change remains paramount. The tension created by this tonal opposition is a defining characteristic of the sonata form.13