The Prussian Quartets are a set of three string quartets, nos. 21–23 (K. 575, 589, 590), nicknamed 'Prussian', composed by Wolfgang Amadeus Mozart between 1789 and 1790 and dedicated to the King of Prussia, Friedrich Wilhelm II. They were posthumously published in December 1791 by Artaria as Op. 18, only a few weeks after Mozart's death.[1] Although not considered on a par with the earlier, more famous Haydn Quartets (K. 387, 421, 428, 458, 464, 465),[citation needed] these pieces are noted for their beauty, equilibrium amongst the instruments and the exceptional prominence and cantabile nature of the cello part (the instrument played by the king himself). These are also the only pieces in this genre that Mozart composed 'to order', i.e. as a commission from a royal patron.