The impression is often given that Mozart, uniquely among the great classical composers, was able to compose easily and quickly without the aid of sketches. True, we have examples of astonishing facility: he is said to have written the overture to Don Giovanni in a single night, and the whole of La Clemenza di Tito in eighteen days. But his mature string quartets certainly cost him a great deal of effort. He was occupied on and off for nearly three years with the six quartets he published as Opus 10 in 1785, and in the dedication to Joseph Haydn he wrote that they were the fruit of a ‘long and very laborious effort’. It was no exaggeration, for an abnormal number of sketches and revisions survive for them; Mozart evidently found it difficult to come to terms with Haydn’s mature string quartet style, as exemplified most recently by the Opus 33 quartets of 1781.