Mozart's friendship with Haydn can be seen in the quartets he published in 1785, dedicated to Haydn at a time where most pieces were dedicated to aristocrats.
Mozart's six quartets dedicated to Haydn (K. 387, K. 421, K. 428, K. 458, K. 464, and K. 465) date from the period 1782 to 1785, and are judged to be a response to Haydn's Opus 33 set from 1781. Mozart met Joseph Haydn in Vienna around 1784, and the two composers became friends. 
