Mozart's friendship with Haydn can be seen in the Haydn quartets he published in 1785, pieces 387, 421, 428, 458, 464, and 465, which were dedicated to Haydn at a time where most pieces were dedicated to aristocrats.
During this time, Haydn and Mozart had become friends, and sometimes played quartets together in Mozart's apartment, with Mozart playing the viola, and Haydn playing violin; see Haydn and Mozart. The "Haydn" Quartets by Wolfgang Amadeus Mozart are a set of six string quartets published in 1785 in Vienna as his Op. 10, dedicated to the composer Joseph Haydn. String Quartet No. 14 in G major, ("Spring"), K. 387, Op. 10, No. 1 (31 December 1782) String Quartet No. 15 in D minor, K. 421/417b, Op. 10, No. 2 (17 June 1783) String Quartet No. 17 in B-flat major ("Hunt"), K. 458, Op. 10, No. 3 (9 November 1784) String Quartet No. 16 in E-flat major, K. 428/421b, Op. 10, No. 4 (JuneJuly 1783) String Quartet No. 18 in A major, K. 464, Op. 10, No. 5 (10 January 1785) String Quartet No. 19 in C major ("Dissonance"), K. 465, Op. 10, No. 6 (14 January 1785) 
