Duo for Violin and Viola no. 1 in G major, K.423

Wolfgang Amadeus Mozart's String Duo No. 1 in G major for violin and viola, K. 423, was written some time between July and October, 1783. It was the first of two duos that Mozart wrote to complete Michael Haydn's set of six for the Archbishop Colloredo. Mozart wrote them as a favour to Michael Haydn, who fell ill before he could complete all six duos. Mozart composed these two duos in the course of just a few days, passing them off as the work of Haydn (although the true authorship was revealed in subsequent newspaper advertisements placed by the Viennese music dealer Johann Traeg). The set of six duos was presented to the archbishop as having all been composed by Haydn, and Colloredo was unable to "detect in them Mozart's obvious workmanship".