Wolfgang Amadeus Mozart, 1756-1791
Piano Trio in B-flat Major, K. 502, 1786
After the string quartet, the piano trio is the most important chamber ensemble since the Classical era, evoking some of the greatest compositions in the literature from nearly every major composer from Haydn to Shostakovich. Its development into a mature form came much later than the string quartet due to at least two important requirements: the emergence of the piano, both the instrument and its technique, sufficient to displace the harpsichord, and, the discovery of effective compositional approaches to blending instruments of unequal tone and dynamic properties in a balanced dialog, namely, the violin, the cello and the piano. Many composers in the late 18th century contributed to a significant body of music for the ensemble including Abel, the Bach sons, Stamitz, Pleyel and Haydn, but these works are all classified as "accompanied sonatas" due to the dominant role of the keyboard (whether harpsichord or fortepiano) and relatively negligent, in some cases optional role of the strings, particularly the cello.
The first mature piano trios of the kind we cherish in the standard chamber music tradition appeared in a close succession of five great works from the mind of a single great composer: Mozart. Written in a two-year period from 1786 to 1788 in Vienna, they culminate in the middle of the series with what are traditionally regarded as his two finest works in the form, K. 502 in B-flat major and K. 542 in E major. As with the six great string quartets Mozart dedicated to Haydn, sketches of the piano trios demonstrate that even Mozart struggled to achieve a satisfactory result. Succeed, he did: debates about which is better aside, both piano trios can be considered virtually perfect chamber music in every sense of the term. Each of the five piano trios comprises three movements deriving more from the piano concerto than either the string quartet or the symphony. With the minuet excluded, the movements present, in order, a sonata, a basic rondo and an extended rondo, establishing a fast – slow – fast tempo scheme.
The first movement of K. 502 has much to teach about the flexibility of sonata form. The second theme of the exposition is different from the first only by virtue of being in a different key and variations of scoring. Unusual for Mozart, but common for Haydn, this "monothematic" type of sonata demonstrates that contrast is not necessarily thematic, but more essentially harmonic. With but a single theme, Mozart crafts his sonata with a second surprise: the development section begins with an entirely new melody, a refreshing contrast of great warmth. This prepares for a dramatic return of the main theme made all the more startling by its new cast in a minor key. Finally, the recapitulation is no mere repeat with a simple harmonic adjustment: as in nearly all of Mozart's sonatas, the return to the beginning brings a new fulfillment of the original music, extended and elaborated into a more perfectly realized and thereby strongly resolved conclusion.
The middle slow movement Larghetto blossoms into music of such exquisite beauty that all words fall mute before its perfect grace. Remove any of the three instruments or even a single note, and the entire structure would fail. The rondo finale wakens from this dream into a new-made world of animated melody, supple motion, taught drama and elegant counterpoint for a deeply satisfying conclusion. Mozart so successfully and delicately balances piano with the violin and cello throughout, that he teaches one more possibly surprising fact about the nature of the piano trio texture: quite often is comprises not three, but four voices, the two hands of the piano decoupled to sing independently from each other, in playful consort with their equally independent stringed cohorts.
