When encountering classical music for the first time, one of the first things that often stands out—sometimes even before the title itself—is the catalogue number. Labels such as Op.23, K.550, or BWV 1043 quickly become familiar, yet the moment we ask why these numbers exist, the subject becomes surprisingly difficult to explain. These combinations of letters and numbers do not describe the mood of the music, its length, or its artistic value. And yet, almost every classical work carries such a designation.
Catalogue numbers are less about the character of the music itself and more about how the music has been recorded, published, and organized over time. Some numbers were assigned for publication purposes, others were created by scholars long after a composer’s death, and in many cases a single work may carry more than one catalogue number at the same time. This article takes a closer look at the different types of catalogue numbers used in classical music, examining why each system emerged and why they continue to coexist today.
1. Opus Numbers – Catalogue Numbers Based on Publication
Opus numbers are the most commonly encountered catalogue numbers in classical music. The term Op. is short for opus, a Latin word meaning “work” or “completed effort.” Originally, it was not intended as a numerical classification system, but rather as a general term referring to a finished composition.
The use of opus as a catalogue number began as music printing and commercial distribution became more widespread. As composers’ works were published, reprinted, and circulated in increasing quantities, publishers needed a practical way to distinguish one publication from another. Opus numbers gradually took on this role as a means of identifying published sets of works.
One crucial point to understand is that opus numbers do not reflect the chronological order in which works were composed. In most cases, opus numbers were assigned not by the composers themselves but by publishers, and a single opus number was often given to an entire group of works released together as one publication.
As a result, multiple pieces can share the same opus number, and works composed around the same time may appear under very different opus designations. Rather than describing the musical content or style of a piece, opus numbers primarily serve as a record of how and when a work entered the world through publication.
Image source: Title page of Beethoven’s Sonata Pathetique, Op.13 (public domain), preserved in Beethoven-Haus Bonn. Wikimedia Commons.
https://commons.wikimedia.org/wiki/File:Beethoven_-_Sonata_Pathetique.jpg
Title page of Beethoven’s Piano Sonata Op.13, published during the composer’s lifetime. The opus number reflects the work’s publication history rather than its compositional order.
2. Works Without Opus Numbers – Music That Was Never Published
When encountering works without an opus number for the first time, beginners often have a natural question. Why do some pieces lack an Op. number and instead carry unfamiliar abbreviations or entirely different catalogues? In these cases as well, the determining factor is not the quality or importance of the music, but whether the work was published.
During the eighteenth and nineteenth centuries, it was not common for every composition to be published immediately. Short pieces written for personal use, educational purposes, or private performances were often left unpublished during a composer’s lifetime. Such works were never included in the opus system and, as a result, remained outside the framework of official publication numbers.
To address this gap, later editors and scholars stepped in. A well-known example is the WoO catalogue created while organizing the works of Ludwig van Beethoven. WoO stands for Werke ohne Opuszahl, meaning “works without opus numbers,” and was designed to group together pieces that had not been published under an Op. designation.
This system is not a supplement to opus numbers, but a separate catalogue with a different point of departure. A WoO designation does not imply that a work is secondary or insignificant; it simply reflects a different publication history. At this point, catalogue numbers reveal their true function more clearly: they are not tools for judging musical value, but markers that explain how a piece of music was recorded, preserved, and passed down.
This example clearly demonstrates that the presence or absence of an opus number has no direct connection to a work’s popularity or lasting significance. The WoO label simply indicates the path through which the piece reached us, remaining separate from the musical value it holds.
3. Catalogue Numbers That Reorganize a Composer’s Entire Output
Opus numbers and WoO designations are both systems rooted in publication history. As interest grew in understanding a composer’s complete body of work regardless of publication status, new attempts were made to reorganize and reassess their output as a whole. From this need emerged composer-specific catalogue numbers, often referred to collectively as catalogue systems.
These numbers were not assigned by the composers themselves. Instead, they were created after a composer’s death by scholars who examined surviving manuscripts, early prints, and historical documents. One important point is that these catalogues were not built on a single shared principle. Depending on what the compiler chose to prioritize, the character of each numbering system differs significantly.
3-1. Catalogues Organized by Chronology
Chronological catalogues were created with the goal of understanding a composer’s creative development over time. Their focus lies in reconstructing when works were written as accurately as possible, and as research advances, these numbers are often revised or refined. Rather than relying on publication records from a composer’s lifetime, such catalogues depend heavily on posthumous scholarly research and manuscript comparison.
K Numbers – Wolfgang Amadeus Mozart
The K numbers assigned to the works of Wolfgang Amadeus Mozart (1756–1791) originate from the catalogue compiled by Ludwig von Köchel (1800–1877). Köchel attempted to arrange Mozart’s compositions in approximate chronological order, and this system remains widely used as a reference for identifying the period in which a work was composed. As research progressed, however, the placement and numbering of certain works were revised, reflecting the fact that the K catalogue is not a fixed outcome but a system shaped by ongoing scholarship.
D Numbers – Franz Schubert
A similar chronological approach can be found in the D catalogue of Franz Schubert (1797–1828), compiled by Otto Erich Deutsch (1883–1967). Like Köchel’s work on Mozart, Deutsch’s catalogue aims to place Schubert’s compositions within a reconstructed timeline. Because many of Schubert’s works were unpublished during his lifetime and sources are often incomplete, the D numbers reflect both the order of discovery and evolving scholarly estimates of composition dates.
3-2. Catalogues Organized by Genre
Genre-based catalogues prioritize the type and function of a work over the time at which it was composed. As a result, the numerical order does not indicate chronology, and works written around the same period are often placed in entirely different sections of the catalogue. The aim of this approach is not to reconstruct a timeline, but to provide a clear overview of a composer’s output by musical category.
BWV Numbers – Johann Sebastian Bach
The BWV numbers used for the works of Johann Sebastian Bach (1685–1750) come from a catalogue compiled by Wolfgang Schmieder (1901–1990). In this system, Bach’s music is grouped by genre—such as cantatas, organ works, instrumental music, and concertos—and the numbers indicate a work’s position within each category. For this reason, BWV numbers are useful for understanding instrumentation and musical character, but they do not directly convey when a piece was composed.
In this way, genre-based catalogues are designed to organize vast bodies of work systematically rather than to reconstruct the passage of time. Although they share the name “work numbers,” they answer fundamentally different questions from chronological catalogues, and this distinction defines their character clearly.
3-3. When Multiple Catalogue Systems Coexist
For some composers, there is no single catalogue system that fully encompasses their works. Numbers created for publication, for scholarly research, and for performance or editorial purposes often originate from different needs, resulting in cases where a single work carries two or more catalogue numbers. In such situations, the difference lies not in the music itself, but in the criteria used to organize it.
K (Kk) Numbers · L Numbers – Domenico Scarlatti
The keyboard sonatas of Domenico Scarlatti (1685–1757) represent one of the most prominent examples of multiple catalogue systems coexisting. The L numbers were compiled by Alessandro Longo (1864–1945), who organized the works with performance and editorial convenience in mind, arranging them according to key and musical character. By contrast, the K (or Kk) numbers were established by Ralph Kirkpatrick (1911–1984) through detailed manuscript research and attempts at chronological reconstruction. Today, the K numbers are generally regarded as the scholarly standard.
Image sources:
1. Parma manuscript collection, Book 2, No.14 (ca. 1752), Biblioteca Nazionale Palatina, Parma, Public Domain.
https://imslp.org/wiki/File:PMLP304748-E535555_63-66–F.Psi.I.48-02-_Sonate_Libro_II.pdf
2. Alessandro Longo (ed.), Opere complete per clavicembalo, Vol.7, G. Ricordi & C., Milan, 1910, Public Domain.
https://imslp.org/wiki/File:PMLP304748-Sonata_K._113_(as_L._345).pdf
3. Domenico Scarlatti, Keyboard Sonata in A major, later edited score with K.113 designation, scan provided by IMSLP, Public Domain.
https://imslp.org/wiki/File:TN-Scarlatti,_Domenico-Sonates_Heugel_32.521_Volume_3_10_K.113_scan.jpg
Three sources showing the same Domenico Scarlatti keyboard sonata under different numbering systems.
The upper image comes from a Parma manuscript, where the number “14” functions only as an internal order within that specific manuscript collection.
The lower-left image represents Alessandro Longo’s editorial system, in which the sonata is catalogued as L.345 according to key and character rather than chronology.
The lower-right image shows a later edition that adopts K.113 as a standardized identifier, reflecting the scholarly catalogue established by Ralph Kirkpatrick.
Together, these sources illustrate that catalogue numbers describe systems of organization, not inherent properties of the music itself.
RV Numbers – Antonio Vivaldi
The works of Antonio Vivaldi (1678–1741) are identified using RV numbers. This catalogue was compiled by Peter Ryom (born 1937) and organizes Vivaldi’s output primarily by type of work, such as concertos, sacred music, and instrumental pieces. Like other genre-based systems, RV numbers do not aim to reflect precise chronological order, but rather to bring structure to a vast and diverse body of compositions.
In this way, the coexistence of multiple catalogue systems is not a source of confusion, but the result of answering different questions. Which number is used depends on the perspective from which a work is approached. Understanding this makes it easier to accept why a single piece may legitimately carry more than one catalogue number.
Chronological catalogues attempt to reconstruct the timeline of a composer’s creative life. However, not all bodies of work lend themselves easily to this approach. For some composers, organizing music strictly by date offers limited insight, leading scholars to adopt different principles when compiling catalogue numbers.
4. Other Frequently Encountered Catalogue Numbers
Beyond the numbering systems discussed above, listeners often encounter several other catalogue numbers when exploring classical music. These numbers, too, were rarely assigned by the composers themselves. Instead, they were created later to organize the extensive output of specific composers in a systematic way. Although the criteria differ from one catalogue to another, they share a common feature: they are scholarly or editorial catalogues rather than publication-based opus numbers.
Hob. Numbers – Joseph Haydn
The Hob. numbers used for the works of Joseph Haydn (1732–1809) come from a catalogue compiled by Anthony van Hoboken (1887–1983). This system first divides Haydn’s music by genre—such as symphonies, string quartets, and piano sonatas—and then assigns numbers within each category. As a result, Hob. numbers are useful for identifying the type of work, but they do not directly indicate when a piece was composed.
S. Numbers – Franz Liszt
The works of Franz Liszt (1811–1886) are identified using S. numbers, derived from a catalogue compiled by Humphrey Searle (1915–1982). This system was designed to organize Liszt’s vast output of piano works, arrangements, and orchestral compositions. Given Liszt’s frequent revisions and reworkings, the S. catalogue plays an important role in clarifying relationships between different versions of the same material.
Op. posth. – Posthumously Published Works
Op. posth. is an abbreviation of opus posthumous, used for works that were published after a composer’s death. Even when an opus number appears in this form, it does not belong to a system that the composer personally approved or managed during their lifetime. This designation is especially common in composers such as Chopin or Mendelssohn, whose unpublished works were issued posthumously, making it important to distinguish between the date of composition and the date of publication.
All of these numbers represent individual organizational approaches rather than parts of a single unified system. For this reason, when encountering a catalogue number, it is often more helpful to consider the principle behind its creation than to focus on the number itself.
Reading Work Numbers
In classical music, work numbers are less a direct reflection of a composer’s intentions than traces of how music has been transmitted, recorded, and organized over time. Opus numbers document publication history, while WoO and various catalogue numbers are the results of posthumous scholarly research. The reason a single work can carry different numbers lies in the fact that music cannot be arranged according to a single, universal principle.
For this reason, a higher number does not necessarily indicate a later work. Some numbering systems are based on chronology, others on genre, and still others reflect only the order of publication. Numbers are not hierarchies of time or value, but outcomes of different methods of classification.
Understanding work numbers is therefore not a matter of memorization, but of recalling the context in which each system was created. When one knows when a piece was written, through what path it was transmitted, and according to which criteria it was organized, the music begins to read not as a mere list, but as a history. At that point, work numbers cease to be confusing symbols and become quiet guides to understanding the music itself.
Further Reading
Chopin New Waltz | A One-Page Discovery from the Morgan Library
Chopin New Waltz | A One-Page Discovery from the Morgan Library