Leopold and his family arrived in The Hague late in January of 1766 to attend the installation of Prince William of Orange as hereditary stadhouder of The Netherlands. This celebration lasted from March 7 to the 12th, and for this occasion Mozart composed the Galimathias Musicum K. 32. It is scored for two violins, two oboes, two horns, obligato Harpsichord, two bassoons, viola and bass. All instruments have solos and the final piece is a fugue for all players based on a Dutch song called "Prince William" which was composed for his Serene Highness, the Prince of Orange. The concluding fugue is in the form of a Quodlibet in that it incorporates various popular tunes. (One famous example of Quodlibet occurs in the last variation of J.S.Bach's "Goldberg Variations").
J. C. Fischer, Mozart met in The Hague at this same time. A melody of Fischer would provide Mozart - in 1774 - the subject for one of his most famous sets of variations for piano. Zwölf Variationen für Klavier über ein Menuett von Johann Christian Fischer K. 179.
Johann Christian Fischer
Return of Mozart's Travels