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
Large numbers If we need many such functions we can better number them instead of using a new letter every time, e.g. as a subscript, so we get numbers of the form formula_81 where "k" and "m" are given exactly and n is an integer which may or may not be given exactly. Using "k"=1 for the "f" above, "k"=2 for "g", etc., we have (10→10→"n"→"k") = formula_82. If "n" is large we can use any of the above for expressing it. Thus we get a nesting of forms formula_83 where going inward the "k" decreases, and with as inner argument a