Three-key exposition
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.
Examples
- A very early example appears in the first movement of Haydn's String Quartet in D major, Op. 17 No. 6: the three keys are D major, C major, and A major.
- 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.
- Felix Mendelssohn followed the Death and the Maiden example in the first movement of his second Piano Trio, in which the E flat major second theme gives way to a G minor close.
- The first movement of Frédéric Chopin's Piano Concerto in F minor also has a three-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.