Mozart could once again make rich use of violas (for Don Giovanni in Prague he had been constrained by an orchestra with only two viola players, but the Burgtheater had a strong section). Anton Stadler and his brother, both virtuoso clarinet players, had joined the orchestra, helping to give respectability to what had previously been a wind-band instrument, and allowing Mozart a glorious palette of orchestral colouring. Così makes sumptuous use of the clarinet.
