The word ‘opus’ is Latin for ‘work’. When it comes to music, the term is included, along with a number, within the titles of individual compositions (or groups of compositions) to help identify a particular piece of work.
For example, Beethoven’s Piano Sonata No. 13 in E-flat major is identified as being linked to the German composer’s Moonlight Sonata by the pieces’ opus number – Opus 27, Number 1 and 2, respectively. The opus number pairs the two pieces together, while the following numbers establish the order of the compositions. Think of it as being similar to a book series with numbered volumes.
Opus is often abbreviated to ‘op’ within the title of a piece, and it should be noted that – due to the haphazard approach of many composers and publishers when it came to using opus numbers – the numbers themselves do not necessarily correlate to the chronological order in which the composers’ work was written or published.
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