Most of us have seen an Opus number attached to a classical piece. But what does this actually mean? Are there other kinds of thematic catalogue numbers, and why?
Opus Number. An Opus number ("opus" being the Latin expression for "work" or "labor"), is typically a way to catalogue a Classical composer's works, most often in chronological order. This can help us to understand if a piece was composed early in a composer's career, or late in their career. Beethoven composed 32 piano sonatas, and they all have the same title. Instead of saying, "I'll be playing Beethoven's Sonata," you can say, "I'll be playing Beethoven's Sonata No. 31 in A-flat, Op. 110," which provides a lot more information!
Deutsch. Franz Schubert didn't only get Opus numbers for his compositions; he was so "extra", he got an additional catalogue for his publications. Deutsch, or D. numbers, are used to identify Schubert's compositions. Schubert helped by dating nearly all of his manuscripts, so putting them in chronological order wasn't quite as complicated as other composers.
K Numbers. Decades after Wolfgang Amadeus Mozart's death in 1791, there were several attempts to catalogue his compositions. It wasn't until Ludwig von Kochel succeeded in producing a comprehensive listing in 1862. Although many of Mozart's earliest works couldn't be put in exact order, only estimated -- Mozart's father, Leopold, had compiled a partial list of his son's earlier works. According to Kochel's counting, Requiem in D minor, K. 626, was the 626th piece that Mozart composed.
BWV. Also known as Bach-Werke Verzeichnis, or Bach Works Catalog in English. Assigned by Wolfgang Schmieder, J.S. Bach's 1,126 compositions were each assigned BWV numbers, grouped by genre: Contatas (BWV 1-224), Motets (BWV 225-231), Masses (BWV 232-243), Keyboard Works (BWV 772-994), and so on.
Longo & Kirkpatrick. For Domenico Scarlatti, the Longo catalogue (L numbers) was in use from 1906, although K numbers (Kirkpatrick, not to be confused with Mozart's Kochel numbers) have generally become a more accepted catalogue.
WoO. Most of Beethoven's works appear with an opus number, but occasionally, you'll see a WoO number. Thank goodness, because it's really just a lot of fun to say out loud like Rick Flair. (Woooo!) Beethoven's WoO numbers (also known as the Kinsky Catalogue) are compiled of his compositions that were not given opus numbers prior to 1955.
There you have it! Of course, many other catalogues exist, but the list above covers many of the heavy hitters and catalogues you should know.
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