A Dictionary of Music and Musicians/Opus
OPUS, OPUS-NUMBER, OPERA, ŒUVRE. A method of numbering musical compositions in the order of their publication, using the Latin word opus (work), began to come into use in the time of Mozart, but was not fully established until Beethoven's time, the numbering not being carried out to all the published works of the former master. No rule is observed as regards the size of an opus: for instance, Beethoven's op. 1 consists of three pianoforte trios, while Schubert's op. 1 is only the song 'Erlkönig.' The opus-number has nothing to do with the date of composition, but only with that of the publication; thus some of Mendelssohn's early works were published (posthumously) with very late opus-numbers. Several mistakes have occurred in the numbering of Beethoven's works in various editions: for instance, the three pianoforte sonatas (op. 31) have often been called 'op. 29,' which is the number of the String Quintet in C, and the last four of the so-called 'posthumous' quartets have been numbered in two different ways. The proper numbering is as follows: the A minor Quartet should be op. 130, not 132; that in B♭ major, op. 131, not 130; that in C♯ minor, op. 132, not 131, and that in F major, op. 133, not 135.
[ J. A. F. M. ]