Opus is Latin for 'work'. Most often the opus number lists the order of date composed or published. Frequently, from the early 1800's a composer's publisher would give numbers to the scores to identify them and keep them apart from the others. The composers of the period would often have different publishers in different cities and each publisher would assign their own opus numbers to the scores. In later years musicologists assigned their own catalog numbers to all or most of a specific composers scores. These works are coded with, usually, the catalogers initials and a number assignment.