Some composers' musical pieces are identified by opus numbers which generally run either in order of composition or in order of publication. Certain composers' works have been definitively cataloged by a given scholar, and in such instances these works can be unambiguously referred to by their thematic catalog abbreviations.
Bach's works are referred to by their BWV or Bachwerkverzeichnis numbers after the catalogue by Wolfgang Schmieder.
Haydn's works are referred to by their Hob or Hoboken numbers after Anthony van Hoboken's 1957 classification.
