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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" (that is, the intermediate) key, Beethoven made various choices: the dominant minor (Piano Sonata No. 2, Op. 2 no. 2; String Quartet No. 5, Op. 18 no. 5), the supertonic minor (Piano Sonata No. 3, Op. 2 no. 3), and the relative minor (Piano Sonata No. 7, Op. 10 no. 3). Later, Beethoven used the supertonic major (Piano Sonata No. 9, Op. 14 no. 1, Piano Sonata No. 11, Op. 22), 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 (translated to D major and minor respectively in the recapitulation). The first movement of the second cello sonata by Brahms also employs a three-key exposition moving to C major and then A minor.
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