Opus

Opus, abbreviated Op., is Latin for ‘work’. As a musical designation it refers to a numbered identifier given to a composition, typically assigned by the composer or publisher in roughly chronological order of composition or publication. Opus numbers are essentially serial designations — Beethoven’s Op. 1, Op. 2, Op. 3, and so on through Op. 138.

Opus numbers help identify and catalog compositions, especially when composers have written many works in similar genres. Beethoven’s ‘Symphony No. 5’ is also ‘Op. 67’ — both designations refer to the same work, but Op. 67 places it in the chronology of his entire output.

Not all composers use opus numbers consistently. Mozart’s works are catalogued by Köchel numbers (K.); Bach’s by BWV (Bach-Werke-Verzeichnis); Schubert’s by D. (Deutsch). These thematic catalogs replace or supplement opus numbers, providing definitive identification.

Latin, ‘work’.

When citing a piece, include the opus or catalog number for unambiguous identification. ‘Beethoven Op. 13’ is more specific than just ‘Beethoven sonata’.