In music, the three-key exposition is a particular kind of exposition used in sonata form. Normally, a sonata form exposition has two main key areas. The first asserts the primary key of the piece, that is, the tonic. The second section moves to a different key, establishes that key firmly, arriving ultimately at a cadence in that key. For the second key, composers normally chose the dominant for major-key sonatas, and the relative major for minor-key sonatas. The three-key exposition moves not directly to the dominant or relative major, but indirectly via a third key; hence the name.
Ludwig van Beethoven wrote a number of sonata movements during the earlier part of his career with three-key expositions. For the "third" key, Beethoven made various choices: the dominant minor, the supertonic minor, and the relative minor. Later, Beethoven used the supertonic major, which is only a mild sort of three-key exposition, since the supertonic major is the dominant of the dominant, and commonly arises in any event as part of the modulation. As he entered his so-called "middle period," Beethoven abandoned the three-key exposition. This was part of a general change in the composer's work in which he moved closer to the older practice of Haydn, writing less discursive and more closely organized sonata movements.
Franz Schubert, who liked discursive forms for the entirety of his short career, also employed the three-key expositions in many of his sonata movements. A famous example is the first movement of the Death and the Maiden Quartet in D minor, in which the exposition moves to F major and then A minor, a formula that is repeated in the final movement; another is the Violin Sonata in A major. His B major piano sonata, D 575, even uses a four-key exposition : this key scheme is literally transposed up a fourth for the recapitulation. The finale of his sixth symphony is an even more extreme case: its exposition passes from C major to G major by way of A-flat major, F major, A major, and E-flat major, making a six-key exposition.
The first movement of the second cello sonata by Brahms also employs a three-key exposition moving to C major and then A minor, the exposition of the first movement of the String Sextet in B flat involves an intervening theme in A major before reaching F, and the Piano Quartet in G minor involves secondary themes in D minor and major respectively. The D minor violin sonata has a final movement that moves through a calm second theme in C major before closing the exposition in A minor.
