(BWV, KV, Deutsch, HWV, etc.) Numerical cataloging systems for organizing a composer's output, usually in terms of chronological creation. These numbers are very helpful in determining exactly which work is called for. For example, Beethoven wrote sixteen (or seventeen, if you count the Grosse Fuge) string quartets. Haydn (opus numbers and Hoboken numbers) wrote over 100 symphonies, all of which are named "Symphony". Tip : Mozart is cataloged with Koechel numbers, J. S. Bach with BWV numbers, Schubert with Deutsch numbers; Vivaldi is cataloged with no fewer than six systems! The point is, supplying the appropriate identifier goes a long way toward getting you the right music. See also WoO .