For Wolfgang Amadeus Mozart, an increasing dissatisfaction with his situation in Salzburg characterized the 1770s.  Battling open antagonism from Archbishop Colloredo and a desire to travel in more cosmopolitan circles, the composer reached a breaking point of sorts in 1777.  After being granted a release from his employment, Mozart travelled with his mother to two extremely important musical centers:  Mannheim and Paris.  Although the journey ultimately did not elicit the commissions Mozart had hoped for, it nevertheless had a profound impact on his composition.  Both Mannheim and Paris had fine orchestras featuring virtuoso players, and the Sinfonia Concertante in E-Flat Major for Violin and Viola illustrates the influence of these encounters on Mozart's composition.  An extremely fashionable genre in both Mannheim and Paris, the sinfonia concertante represented an innovation that Mozart was quick to embrace—perhaps, as one scholar suggests, to stage a musical protest against what he viewed as the Archbishop's provincial and confining attitudes towards music. 

Though the Sinfonia Concertante is sometimes called a "double concerto"—in the spirit of a work like Johannes Brahms's Double Concerto for Violin and Cello, perhaps—the two terms were not interchangeable in the 18th century.  While a concerto focuses on the dynamic relationship between soloist and orchestra, the sinfonia concertante highlights the interplay among soloists.  Furthermore, the musical motives that the soloists present in a sinfonia concertante are largely independent of those introduced by the orchestra.  (Mozart's sinfonia concertante observes this tutti-solo distinction markedly; virtually none of the principal themes from the lengthy orchestral introduction returns in the solo exposition).  The result is dramatically rich. The main action occurs in the dialogues between the violin and the viola, while the orchestra functions as a sort of Greek chorus, commenting on the drama that has transpired without ever entering into it directly.   

Indeed, the violin and the viola share center stage in all three movements of K. 364, becoming not only its featured performers, but its sole protagonists.  The cadenzas, for example—written by Mozart himself—seem almost like soliloquies that proceed naturally from the drama, rather than isolated instances of frantic virtuosity.  In the achingly beautiful second movement, the instruments are disguised as singers in an opera, trading luxuriously lyrical lines before joining together in a magnificent, harmonious climax. Further distinguishing the soloists from the orchestra is Mozart's treatment of the solo viola.  Written with scordatura tuning (raised a half-step), the solo viola part offers an audibly brighter tone than its orchestral counterparts.  Throughout the work, the influence of the Mannheim orchestra is audible in gestures such as the opening dotted rhythms and the trademark “Mannheim crescendo” over an eighth-note pedal point that ends the first orchestral section.