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
Rencontres numbers Rencontres numbers In combinatorial mathematics, the rencontres numbers are a triangular array of integers that enumerate permutations of the set { 1, ..., "n" } with specified numbers of fixed points: in other words, partial derangements. ("Rencontre" is French for "encounter". By some accounts, the problem is named after a solitaire game.) For "n" ≥ 0 and 0 ≤ "k" ≤ "n", the rencontres number "D" is the number of permutations of { 1, ..., "n" } that have exactly "k" fixed points. For example, if seven presents are given to seven different people, but only two are destined to