Opus number

In musical composition, the Opus number is the "work number" that is assigned to a composition, or to a set of compositions, to indicate the chronological order of the composer's production. 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.

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 in C-sharp minor (1801) (nicknamed Moonlight Sonata) is "Opus 27, No. 2", which work-number identifies it as a companion piece to "Opus 27, No. 1" (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 N° 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 assignment of opus numbers, especially during the Baroque era (1600–1750) and the Classical era (1750–1827), musicologists developed other catalogue-number systems; among them the Bach-Werke-Verzeichnis (BWV-number), and the Köchel-Verzeichnis (K- and KV -numbers) with which are organised the works of Johann Sebastian Bach and Wolfgang Amadeus Mozart, respectively.

In the classical period, the Latin word opus ("work", "labour") was used to identify, list, and catalogue a work of art.

By the 15th and 16th centuries, the word opus was used by Italian composers to denote a specific musical composition, and by German composers for collections of music. In compositional practise, numbering musical works in chronological order dates from 17th century Italy, especially Venice. In common usage, the word Opus is used to describe the best work of an artist with the term magnum opus.

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