An opus number is a number attached to a particular musical composition or set of compositions by a publisher to denote the order of publication by the given composer. Theoretically, opus numbers would give musicologists a simple means to chart the growth of a composer chronologically, to make statements to the effect that there is a direct correlation between such and such composer’s handling of a particular musical technique and the opus numbers.
In practice, however, many factors contribute to making the opus numbers for certain composers less than useful, such as their being assigned by the composer rather than the publisher. Antonín Dvořák, for example, tried to fool publishers in an effort to get the best deal, and thus often gave misleadingly low or high opus numbers to his compositions. The confusion with Carl Nielsen’s opus numbers arose not from any wheeling and dealing, but from his not having an exclusive publisher and not keeping track of which opus numbers he hadn’t used (see sequence A113529 in Sloane’s OEIS for a listing of opus numbers skipped over by Carl Nielsen).
Other composers just never had any opus numbers given to their compositions by either publishers or themselves; musicologists then sometimes try to create a catalog with numbers named after themselves. In the case of Mozart, the Köchel numbers by Ludwig von Köchel are almost universally accepted, but in the case of Domenico Scarlatti, three different musicologists came up with three different catalogs, leading to many of Scarlatti’s sonatas to be known by three different numbers. The lack of opus numbers is not a problem for composers whose repertory works consist of less than a dozen of large-scale works (e.g., Bruckner and Mahler).
- 1 Schönzeler, H. H. Dvořák London: M. Boyers (1984): 220 - 239