This ''No. 1'' of Mozart's eighteen total piano sonatas is nevertheless not the very first one that he composed. It is well known that the ''Wunderkind'' Mozart had previously written at least four piano sonatas (K. Anh. 199-202), though these have never surfaced. And the sequence 1-6 of the piano sonatas K. 279-284, which were probably composed in Munich at the beginning of 1775, is based solely on Mozart's retroactive numbering of them (with just the manuscript of the first movement of the first sonata having been lost). In the family correspondence these six sonatas are, incidentally, characterized many times as the ''difficult sonatas.'' That can scarcely be meant in a technical sense, but, as the editor of the Henle Urtext edition, Ernst Herttrich, surmises, might rather have alluded to their enormous ''aesthetic, interpretive demands,'' given that ''Mozart furnished ů the six sonatas with exceptionally rich dynamic and articulation markings ů more than any of his previous works.''