Opus Number (Op.)
The publisher's work number — one opus can hold several pieces and need not follow composition order
Related
Op. (opus, plural opera) is Latin for "work", and in practice the publisher's number for a work or a set. It is usually assigned when the music is printed: Beethoven's Op. 18 is a set of six string quartets, and one of them is cited as Op. 18 No. 1.
The key is that publication is not composition. A piece written early may appear late and receive a high number; publication order and composition order do not match. Works issued after the composer's death are often marked Op. posth., and different publishers can give the same piece different opus numbers.
So Op. is neither a chronology nor a complete catalogue; it records a batch sent to the printer. Beethoven's Op. 1 is a set of three piano trios, Op. 135 his last quartet — the number follows publication, not the life.
Read deeper → WoO · Deutsch Number
Sources
- Op. 的出版性质与编号惯例，参 The New Grove Dictionary of Music and Musicians, "Opus"；本文为原创表述
Used for fact-checking only; all prose is original.