Mozart's Divertimento in D major, K. 136 (K.125a) is the first of a group of works collectively known as the "Salzburg" symphonies. These works stands apart from Mozart's remaining symphonies, in that they are set for strings alone, rather than for the otherwise customary mixed instrumentation including winds. A further point which separates these compositions from Mozart's others in the symphonic genre, is that they are comprised of just three, rather than four individual movements, each lacking the usual Minuet. And lastly, the compact three-movement form further distinguishes the "Salzburg" symphonies from Mozart's true Divertimentos and Serenades, which were mulit-movement creations on a large scale, regularly spanning six movements and sometimes even more.
In keeping with Classical conventions, works such as these for string orchestra could also be played by the four voices of the string quartet when the occasion demanded. Indeeed, a likely explanation for the origin of these so-called "Quartet Divertimentos" is to be found in the facts of Mozart's life at the time of composition, early in 1772. Mozart was then just 16 years of age, and already held the post of Court Concertmaster to Hieronymus Coloredo, Prince Archbishop of Salzburg. This period fell between Mozart's second and third visits to Italy, where he may well have found the impulse to compose works in the style of the three-movement Sinfonias and Concerti Grossi which had been popular since the times of Corelli. This, however, is still a matter of considerable conjecture, and besides, had these been pieces to be played exclusively by solo instrumentalists, Mozart surely would have considered them as true string quartets.
Mozart's Divertimento (125a) seems indeed to closely mirror the style of the Italian concertos for strings, which he must certainly have encountered during his several visits to Italy. The work consists of a lively opening Allegro, in simple sonata form; a charming central Andante; and a brilliant concluding Presto. It is interesting to compare this work with its close companion in B flat, K. 137, which follows a slightly different general scheme, in which the main Allegro is placed second, and follows (unusually) a first movement headed "Andante." The brilliant inventiveness and virtuosity of the D major Divertimento is, to echo the words of Alfred Einstein (writing about another closely related work, Mozart's perennial Eine kleine nachtmusik, the Serenade in G, K. 525) "a masterpiece of masterpieces, on the smallest possible scale."