21 in D major, K. 575, was written in June 1789 by Wolfgang Amadeus Mozart.
It has acquired the nickname The Violet, used for example in Hans Keller's chapter of The Mozart Companion.
There are four movements: The quartet was written for (and in Mozart's private 'thematic catalogue') was described as being dedicated to the King of Prussia,[1] Friedrich Wilhelm II, an amateur cellist.
Mozart and his friend Karl Lichnowsky met the king in Potsdam in April 1789.
The middle part of the song "Mozart's House" by UK based indie electronic band Clean Bandit samples the piece.