Have you noticed that the name some pieces of music have a series of letters and numbers at the end? Symphony 41, K. 551; Moderato, Op. 41; Orchestral Suite in b Minor, BWV 1067. They each have a different set of letters and numbers at the end – what does that all mean?

In each case, the letters and numbers represent a way of cataloging the works composed by a specific composer. In some cases a historian organized and numbered all compositions after a composer died. Sometimes the letter(s) prior to the number represent the historian who did the cataloging.

A general way of cataloging works of a composer is to use an Opus number. Opus numbers may be used for multiple composers; the opus numbers are sequential for a single composer. For example, there may be an Opus 33 for Joachim Andersen and an Opus 33 for Franz Joseph Haydn.

Unlike opus numbers, specific catalogues usually contain works of a single composer, and therefore each number is unique. For example, Kochel numbers are used for the music of Mozart, so there is only one K. 33.

Here are some of the most common cataloguing indicators.

Opus (Op) Numbers

An Opus number represents a chronological order of pieces written by a specific composer. The chronology may be the order in which the pieces were composed, or it may be the order in which they were published, depending on the composer and the person who did the numbering.

Opus is abbreviated Op., such as 24 Etudes for Flute, Op. 15

BWV Numbers

The Bach-Werke-Verzeichnis (Bach works catalogue) assigned a number to every piece of music composed by Johann Sebastian Bach. It was published in 1950 and grouped compositions by genre (e.g., all cantatas are in a sequentially numeric block).

Hob Numbers

The Hoboken catalogue includes the works of Franz Joseph Haydn as compiled by Anthony van Hoboken. This catalogue was started in 1934 and the final publication was in 1978. Since this catalogue includes over 700 works, the numbering system includes a Roman number indicating the genre and an Arabic number indicating the number within that genre, separated by a colon (e.g., X:11)

HWV Numbers

The Händel-Werke-Verzeichnis series of numbers designates the catalogue of the works of George Frederick Handel. This catalogue was published from 1978 through 1986 and includes the first several bars of each piece and some information about the work.

K or KV Numbers

The Kochel catalogue contains all compositions of Wolfgang Amadeus Mozart. This catalogue was originally created by Ludwig von Kochel in 1862 (during the American Civil War). He attempted to catalogue all of Mozart’s works in the order in which they were composed, but some of the early works can only be estimated.

WoO Numbers

The Works without Opus Numbers catalog (WoO) was created in Germany during the 1950s by Georg Kinsky and Hans Halm. This catalog focuses on the works of Beethoven, but WoO numbers are also sometimes used with other composers.