As part of the CSO From Home video series, violin Rong-Yan Tang and viola Youming Chen of the Chicago Symphony Orchestra perform the first movement of Mozart’s Duo for Violin and Viola in G Major, K. 423. Written in 1783, as a favor to an old friend, the work treats the viola as a full partner in the musical discourse. Small wonder: though Mozart played both violin and viola, he reportedly favored the latter.