Opus number: definition, history, and how musical works are identified

Meaning and typical use

In printed music and program notes, an opus number serves primarily as an identifier rather than a strict biography of composition dates. Composers and publishers applied opus numbers unevenly: some composers gave numbers themselves, others had publishers assign them, and still others never used them. For example, when discussing keyboard music it is common to attach an opus to a set of piano pieces or to groups of sonatas. References to a composer such as a composer plus an opus number are meant to pinpoint a particular work for study, performance, or cataloguing.

History and limitations

Opus numbering became more regular as music publishing grew in the 18th and 19th centuries, but it originally applied only to works that appeared in print. Before and around the end of the 18th century, many compositions circulated in manuscript or were unpublished, and therefore lacked opus numbers. That practice produced inconsistencies: an important early work might lack an opus number while later, less significant pieces did have one. Publishers sometimes grouped several pieces under a single opus number, further complicating chronological assumptions.

Catalogues and alternatives

Because opus numbers are incomplete or inconsistent for many composers, musicologists developed comprehensive catalogues that assign each work a unique identifier. A familiar example is the Köchel catalogue for Mozart, where pieces are numbered with a K (or KV) prefix. This approach lets scholars distinguish items such as two distinct symphonies in the same key: a Symphony in G minor may appear with a Köchel number rather than an opus number. Similar specialist catalogues exist for many composers; these are often compiled by musicologists who study manuscripts, publication history and stylistic evidence to produce a reliable listing.

Practical examples and special cases

• Some composers, like Beethoven, are commonly cited by opus numbers for many works; opus numbers can indicate a general place in a career but are not always strictly chronological.
• Other composers, such as Mozart, lack traditional opus numbers in modern references and are instead cited with catalogue numbers (e.g., Köchel numbers).
• Posthumous works, juvenile pieces, fragments and unpublished items are sometimes labelled with prefixes such as "WoO" (without opus) or assigned catalogue numbers by later editors.

Notable distinctions and terminology

In English the plural form is commonly "opuses" rather than the Latin plural "opera," because "opera" is already an independent musical term. For clarity, discussions about a body of work may use "opus" to mean a single piece or, colloquially, an artist’s entire achievement (e.g., someone’s "magnum opus"). When reading scores, programs or scholarly texts, check whether the reference is to an opus number assigned by the composer, one assigned by a publisher, or a later catalogue number; each conveys different information about provenance and chronology.

For further reading about notation practices, historical publishing and common catalogues, consult specialized music history resources and the authoritative lists of individual composers compiled by scholars and libraries. See also examples of how publishing practices affected numbering in the 18th and 19th centuries and the treatment of multi-piece publications as single opus entries (examples in key signatures). Additional context on operatic and vocal work terminology is available through general references (operas) and language sources (Latin usage and the plural form opera).