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
Köchel catalogue and Georges de St. Foix) reassigned a number of works from the original K appendices into the main catalogue by interpolating new numbers into the main sequence with a lower-case letter suffix. In K some of these were reassessed in the light of scholarship since 1937 and returned to the re-worked appendices: For example, the Divertimento for Wind Octet in E♭ was numbered Anh. 226 in K; Einstein placed it in the K main catalogue as K. 196e, between K. 196 and K. 197; K reassigned it again to the 'doubtful' appendix C as Anh. C 17.01. Some works in