Mozart lived at the center of the Viennese musical world, and knew a significant number and variety of people: fellow musicians, theatrical performers, fellow Salzburgers, and aristocrats, including some acquaintance with Emperor Joseph II. Solomon considers his three closest friends to have been Gottfried von Jacquin, Count August Hatzfeld, and Sigmund Barisani; others included his elder colleague Joseph Haydn, singers Franz Xaver Gerl and Benedikt Schack, and the horn player Joseph Leutgeb. Leutgeb and Mozart carried on a curious kind of friendly mockery, often with Leutgeb as the butt of Mozart's practical jokes.