Three-key exposition

The three-key exposition is a variation of sonata form where the exposition moves through three keys instead of the usual two. Typically, a sonata exposition establishes the tonic and then moves to either the dominant (for major keys) or the relative major (for minor keys). In contrast, the three-key exposition introduces an intermediate key before reaching the final key.

Examples include Haydn's String Quartet in D major, Op. 17 No. 6, which uses D major, C major, and A major. Beethoven experimented with three-key expositions early in his career, using keys such as dominant minor, supertonic minor, or relative minor, but later abandoned this approach during his "middle period." Schubert frequently employed three-key expositions, as seen in works like the Death and the Maiden Quartet, which transitions through D minor, F major, and A minor. Some of Schubert's compositions even use four or six keys, such as his B major piano sonata (D 575) and the finale of his sixth symphony (D 589).

Mendelssohn followed Schubert's example in his second Piano Trio, while Chopin used a three-key exposition in his Piano Concerto in F minor. Brahms also incorporated this structure in several works, including his second cello sonata and string sextet. The three-key exposition offers composers a more expansive harmonic framework within the traditional sonata form.