In 1781 Joseph Haydn published his six “Russian” Quartets, Op. 33, which revolutionized the medium, previously largely a matter of first violin plus accompaniment. The democratic involvement of all four instruments in thematic give-and-take and the liberation of thematic elaboration from confinement in the “development” section of a movement was a radical breakthrough, and one quite congenial to Mozart, newly arrived in Vienna after his own personal liberation from Salzburg.

Mozart began work on six quartets of his own, which he completed in January 1785. He dedicated the set to Haydn, writing: “Behold here, famous man and dearest friend, my six children. They are, to be sure, the fruit of long and arduous work, yet some friends have encouraged me to assume that I shall see this work rewarded to some extent at least, and this flatters me into believing that these children shall one day offer me some comfort. You yourself, dearest friend, have shown me your approval of them during your last sojourn in this capital.”

The sixth of these “Haydn” Quartets, No. 19 in C, K. 465 was nicknamed “Dissonant” because of its fascinatingly chromatic Adagio introduction, which finally explodes into a dramatically driven Allegro in bright C major. (One wonders how much influence this had on Haydn, when he came to put down his famous depiction of chaos at the beginning of his oratorio The Creation, which also lands in radiant C major.) The middle movements contrast the elegant eloquence of the Andante cantabile – truly a “singing” movement – with an uncommonly bumptious Minuet. The Finale is a thoroughly Mozartian riff on the fleet, tricky, and witty finales of Haydn.