What is the significance of opus numbers?
Opus is the Latin word for “work,” and is abbrieviated Op. Opus numbers give an indication of the chronological order of a composer’s works, although they often reflect the order of publication rather than the exact order of composition. Sometimes a group of related pieces is published as a set with one opus number, in which case each piece is given an additional number. Beethoven’s string quartet Op. 59, No. 3, for example, is the third of three string quartets published as Op. 59. Beethoven was in fact the first major composer to assign opus numbers to his own works. The works of some composers are identified by catalog numbers rather than opus numbers. Mozart’s works, were catalogued by a man named Ludwig Kochel, and so they all bear “K” numbers.
