Opus
A work, counted as an op. number.
What it means
Opus means “work” and designates, with an attached number, a work within a composer's numbering — abbreviated op., in the plural opp. The number serves to distinguish: it makes clear which of several pieces of the same genre and key is meant.
Musical use
Opus numbers, as a rule, do not depict a sequence of composition. They were usually assigned at publication, and the order of the printings did not correspond to the order of composition: youthful works sometimes appeared late, late works early, and some numbers were assigned by publishers without consultation or used twice. Works not printed during the composer's lifetime often carry no opus number at all; for them the designation “without opus number” has become established. An opus number can, moreover, comprise several pieces published as a collection, and is then subdivided by a second number.
Because of this unreliability, dedicated catalogues of works have become established for numerous composers, ordering by genre or by established chronology and working with their own abbreviations. Today these are the more reliable reference, and scholarly literature usually cites by them.
For practice this means: whoever wants to name a work unambiguously names genre, key, and catalogue number; the opus number alone can be ambiguous.
The opus number counts works in the order of their publication, not always of their composition. Publishers often assigned the numbers, and with some composers they are incomplete or assigned twice. This is why dedicated catalogues of works exist for many composers, with abbreviations such as BWV for Bach, KV for Mozart, or D for Schubert. Works without an opus number are designated WoO.
Performance practice
When ordering or in programme information, check whether the designation is unambiguous — with collections, the internal number belongs with it. Do not draw conclusions about maturity or date of composition from an opus number; the catalogue of works is responsible for that. And watch, with editions, which numbering they use, so that in ensemble playing two different pieces are not meant.