Opus (Latin sing. "work", "labour", a work of art, 1695–1705), pl. opera and opuses, abbreviated, sing. Op. and pl. Opp. The Latin plural, opera, also denotes the opera music genre, wherein the works also are (occasionally) identified with a musical composition opus number, as in the grand opera Samson and Delilah, Op. 47, by Camille Saint-Saëns.
In the arts, opus number usually denotes a work of musical composition, a practice and usage established in the seventeenth century when composers identified their works with an opus number. In the nineteenth century, publishers usually assigned opus numbers when publishing groups of like compositions, usually in sets of three-, six-, and twelve compositions. Consequently, opus numbers usually are not chronologic, unpublished compositions usually had no opus number, and numeration gaps and sequential duplications occurred when publishers issued contemporaneous editions of a composer’s works, as in the sets of string quartets by Joseph Haydn (1732–1809) and Ludwig van Beethoven (1770–1827); Haydn’s Op. 76, the Erdödy quartets (1796–97), comprises six discrete quartets consecutively numbered Op. 76 No. 1 – Op. 76 No. 6; whilst Beethoven’s Op. 59, the Rasumovsky quartets (1805–06), comprises String Quartet No. 7, String Quartet No. 8, and String Quartet No. 9.
19th century to date
From about 1800, composers, especially Ludwig van Beethoven, assigned an opus number to a work, and later to a set of works, especially songs and short piano pieces; however, composers’ inconsistent usages ended the correspondence between an opus number and the work’s publication date. Since approximately 1900, composers tended to assign an opus number to a composition, published or not. Early in his career, Beethoven selectively enumerated his compositions (some published without opus numbers), yet in later years, he published early works with high opus numbers. Likewise, some posthumously published works were given high opus numbers by publishers, even though some of them were written early in Beethoven's career. Since his death in 1827, the un-numbered compositions have been catalogued and labelled with the German acronym WoO (Werk ohne Opuszahl), meaning "work without opus number". However, there are other catalogues of Beethoven's works - see Catalogues of Beethoven compositions.
The practice of enumerating a posthumous opus (Op. posth.) is noteworthy in the case of Felix Mendelssohn (1809–47); after his death, the heirs published many compositions with opus numbers Mendelssohn did not assign them. In life, he published three symphonies, Symphony No. 1 in C minor, Op. 11; Symphony No. 2 in B flat major, Op. 52; and Symphony No. 3 in A minor, Op. 56, yet, he chronologically wrote symphonies between symphonies Nos. 1 and 2, which he withdrew for personal and compositional reasons; nevertheless, the Mendelssohn heirs published (and catalogued) them as the Italian Symphony No. 4 in A major, Op. 90 and as the Reformation Symphony No. 5 in D major and D minor, Op. 107.
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