The Beethovenian concept of “early”, “middle” and “late” applies fruitfully to Mozart’s string quartets. Of the 23 he wrote, a full 13 comprise the early period, the epic 6 quartets dedicated to Haydn the middle, with a final 4 quartets suggesting a late period. The difference in each case is refinement, particularly of texture.
The last 3 are known as the “Prussian” quartets as they were likely intended for the cellist Frederick Wilhelm II, King of Prussia, the musical monarch who inspired works from Haydn and Pleyel among others. In the quartets from all three composers, the cello enjoys a prominent role, music intended for the king to actually play. Mozart never managed to finish a full set of six that he mentions in his letters nor did he ever submit them to the king: desperate for cash, Mozart hurriedly sold the set of 3 completed quartets to a Viennese publisher. These would be Mozart’s last quartets written within a year or so of his death, published posthumously.
In these “late” quartets, Mozart reaches a level of refinement, grace, contrapuntal finesse and rarefied textures featuring melody more than motive, sublime nuance more than dramatic gesture. Particularly in the first quartet, K. 575 in D, the cello rises as prominent soloist especially in the minuet trio and the finale. The texture features each of the instruments as concertante soloist yet still interwoven into an organic dialog of great symmetry and balance. Mozart’s mastery of opera is evident throughout these rich quartets with lyrical exchanges and delicate, but finely spun “accompaniments.” High registers, sparse, “colorful” textures and an elegant ease of gallant melodiousness grace these “Prussian” quartets with a nearly miraculous sense of transcendence suggestive of passages in Beethoven’s future, late quartets. André Gide once wrote, “Of all musicians Mozart is the one from whom our epoch has taken us farthest away; he speaks only in a whisper, and the public has ceased to hear anything but shouts.”
Mozart's final string quartets comprise a set of three collectively known as the "Prussian" or the "Berlin" quartets. In 1789, friend and student Prince Karl Lichnowsky took Mozart to Berlin to meet Frederick William II, King of Prussia (the second son of Frederick the Great). Frederick was a skillful cellist, and a generous patron of the arts. The meeting proved fruitful for Mozart resulting in a commission for six string quartets as well as some piano sonatas for Frederick's daughter. But these final years were difficult times for Mozart. His letters paint of picture of illness, his wife's difficult fifth pregnancy, debt and urgent pleas for yet more loans from overly taxed friends. Mozart completed the first quartet straight away, spent nearly a year working on Così fan tutte, then managed to complete two more quartets in May and June of 1790. Financial desperation ultimately forced Mozart to monetize his latest work as swiftly as possible: he sold the three quartets to the Viennese publisher Artaria who released them in print shortly after Mozart's death in 1791 without any dedication to the Prussian patron. 
