opus
A number assigned to a composer's works, usually in order of publication.
In Depth
Opus numbers (abbreviated Op.) are assigned to a composer's works, usually in order of publication rather than composition. They serve as a catalogue system, allowing musicians and scholars to identify specific works precisely — Beethoven's Op. 27, No. 2 is the Moonlight Sonata.
The opus numbering system is not always reliable. Some works were published out of order, and many composers' early or unpublished works received opus numbers posthumously. Alternative cataloguing systems exist for major composers: Bach's works use BWV numbers, Mozart's use K (Köchel) numbers, and Schubert's use D (Deutsch) numbers. These specialist catalogues are often more accurate than opus numbers.
Beethoven's Op. 1 consists of three piano trios he published at age 25, though he had been composing for over a decade. He waited until he had works worthy of an official debut.