opuses

An opus is a term used in music and literature to designate a work produced by a single author or composer, especially when published in a numbered sequence. The word comes from Latin opus, meaning "work" or "production," and entered common usage in European art music in the early modern period.

In classical music, Op. stands for opus and is usually followed by a number, sometimes accompanied by a subdivision such as "No. 2" or "Nos." The sequence generally reflects publication order, but it does not always correspond to the date of composition. Some composers published works without opus numbers, while others released works posthumously with assigned opus numbers. In many cases, a composer also has a separate catalog or thematic index (for example, Bach's BWV, Mozart's K., or Schubert's D.) that serves a different purpose than opus numbering.

Opus numbers are per-work identifiers rather than a formal universal system; they help distinguish works with similar titles and organize a composer's outputs. When multiple pieces appear under the same opus, subnumbers such as "No. 1" and "No. 2" are used. Examples include Beethoven's Moonlight Sonata (Piano Sonata No. 14 in C-sharp minor, Op. 27, No. 2) and Debussy's La Mer (Orchestral Suite, Op. 22). Chopin's Etudes are collected as Op. 10 and Op. 25, etc.

The plural of opus in English is usually opuses; opera is the Latin plural and not typically used to refer to multiple opus works in English. Opus as a concept remains central to musicology and cataloging, though it coexists with alternative systems that aim to reflect a composer’s complete output more comprehensively.