Editing Opus number (section)

{{Use dmy dates|date=
September 2021}}
{{short description|Number indicating the chronological order of a composition among all of the composer's works}}
{{refimprove|date=October 2010}}

In [[musicology|music]], the '''opus number''' is the "work number" that is assigned to a [[musical composition]], or to a set of compositions, to indicate the [[chronological order]] of the [[composer]]'s publication of that work. Opus numbers are used to distinguish among compositions with similar titles; the word is abbreviated as "Op." for a single work, or "Opp." when referring to more than one work. Opus numbers do not necessarily indicate chronological order of composition. For example, posthumous publications of a composer's [[juvenilia]] are often numbered after other works, even though they may be some of the composer's first completed works.

To indicate the specific place of a given work within a [[music catalogue]], the opus number is paired with a [[cardinal number]]; for example, [[Beethoven]]'s [[Piano Sonata No. 14 (Beethoven)|Piano Sonata No. 14 in C-sharp minor]] (1801, nicknamed ''Moonlight Sonata'') is "Opus 27, No. 2", whose work-number identifies it as a companion piece to "Opus 27, No. 1" ([[Piano Sonata No. 13 (Beethoven)|Piano Sonata No. 13 in E-flat major]], 1800–01), paired in same opus number, with both being subtitled ''Sonata quasi una Fantasia'', the only two of the kind in all of Beethoven's 32 piano sonatas. Furthermore, the ''Piano Sonata, Op. 27 No. 2, in C-sharp minor'' is also catalogued as "Sonata No. 14", because it is the fourteenth [[sonata]] composed by Ludwig van Beethoven.

Given composers' inconsistent or non-existent assignment of opus numbers, especially during the [[Baroque music|Baroque]] (1600–1750) and the [[Classical music era|Classical]] (1750–1827) eras, [[musicologists]] have developed other catalogue-number systems; among them the ''[[Bach-Werke-Verzeichnis]]'' (BWV-number) and the ''[[Köchel catalogue|Köchel-Verzeichnis]]'' (K- and KV-numbers), which enumerate the works of [[Johann Sebastian Bach]] and [[Wolfgang Amadeus Mozart]], respectively.