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. Historically, although composers have inconsistently applied the opus number to their works, besides numeric cataloging, opus numbers are used to distinguish among compositions with similar titles.

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.