Opus number

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In music, the opus number is the "work number" that is assigned to a musical composition, or to a set of compositions, to indicate the chronological order of the composer's publication of that work. Opus numbers are used to distinguish among compositions with similar titles; the word is abbreviated as "Op." for a single work, or "Opp." when referring to more than one work. Opus numbers do not necessarily indicate chronological order of composition. For example, posthumous publications of a composer's juvenilia (written as "Op. posth.") are often numbered after other works, even though they may be some of the composer's first completed works. To indicate the specific place of a given work within a music catalogue, the opus number is paired with a cardinal number; for example, Beethoven's Piano Sonata No. 14 in C♯ minor (1801, nicknamed Moonlight Sonata) is "Opus 27, No. 2", whose work-number identifies it as a companion piece to "Opus 27, No. 1" (Piano Sonata No. 13 in E-flat major, 1800...

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