Opus number developed other catalogue-number systems; among them the "Bach-Werke-Verzeichnis" (BWV-number), and the "Köchel-Verzeichnis" (K- and KV -numbers) with which are organised the works of Johann Sebastian Bach and Wolfgang Amadeus Mozart, respectively. In the classical period, the Latin word "opus" ("work", "labour") was used to identify, list, and catalogue a work of art. By the 15th and 16th centuries, the word "opus" was used by Italian composers to denote a specific musical composition, and by German composers for collections of music. In compositional practise, numbering musical works in chronological order dates from 17th century Italy, especially Venice. In common usage, the
Catalogues of Beethoven compositions and Hans Halm published a catalogue of Beethoven's works, in which they assigned numbers to 205 "Werke ohne Opuszahl" (meaning "works without opus number" in German) to "some" of Beethoven's unpublished works. These numbers given these works are generally preceded by "WoO". The Kinsky–Halm catalogue also lists 18 works in an appendix as being either doubtfully attributed, or simply spurious. Some of those listed as doubtful have since been shown to be authored by Beethoven. These numbers are generally preceded by "Anh.", short for German "Anhang", meaning "appendix". The Swiss composer and musicologist Willy Hess also researched and published a