What do the following have in common?

• Schönberg – Piano Concerto,
• Schumann – song cycle Frauenliebe und -leben,
• Shostakovich - Five Fragments for Orchestra, and
• Sibelius – Romance for Strings in C.

Well, apart from the blindingly obvious – the composers' names all begin the letter S – they are the composers' respective Opus 42¹ works. But what exactly is an opus number? Does the composer put Op.1 on the first piece of music he or she writes, then Op.2 on the next one and so on throughout their lifetime? And then why is there no work by Mozart marked Op. 42? The question, what is an opus number, is therefore a legitimate one to ask, but unfortunately, one that does not have a simple, straightforward answer.

The word Opus is a Latin singular noun, meaning work. Strictly speaking, its plural should be Opera, but given that in Italian, the word Opera (ie work) has become the singular and Opere the plural, to avoid confusion, normal convention is to adopt as the plural the term opuses. The singular abbreviation is Op. (Op.47) and for multiple works, Opp. (Opp.27-31).

As a general rule, the opus number represents a work's approximate position in the timeline of a composer's musical output – works with lower opus numbers generally being written before works with higher opus numbers – but there are many exceptions to this general rule. Knowing where a musical composition belongs in the sequence of a composer's output is useful in understanding its context. Is it a youthful, exuberant work, or a later, and therefore more mature, reflective work?

The Czech composer, Antonin Dvorák, allowed his publisher to give higher opus numbers to some of his earlier works, an example of where the opus number does not give a useful guide to the chronology of a composer's work.

Why do we need an opus number?

Clearly for some works they are not strictly needed; for example the Symphony No. 2 by Brahms uniquely differentiates it from that, or indeed any other composer's First or the Third symphonies. It would not be unreasonable to assume that it was written after the First and before the Third, so we satisfy our timeline requirement. What about using the year of composition? The first of many problems with using dates is that very often they are not known exactly. We could perhaps use the composition's home key – always assuming that the work is tonal – but we find for instance that among Beethoven's String Quartets, three are in the same key, F major². How should these be differentiated?

Who started this?

So it's the composer who assigns opus numbers to his work is it? Well, it can be, but it is far more likely to be assigned by the music publisher. The first recorded appearance of an opus number was in the second half of the 15th Century. It was used by German publishers in the 16th Century, often for collections written by several composers. Pre-1800, it was more commonly used for instrumental, rather than vocal works; it has rarely been used for works intended for stage performance – especially operas and ballet – at any time.

To add further to the confusion, the same composition made available from different publishers could carry different opus numbers! In times when compositions were quite short, it was common practice to issue several short compositions in one publication. As works started to grow in length, this practice reduced, although it continued for groups of string quartets. For example the Beethoven set of six quartets, Op.18, are referred to as String Quartet in F, Op.18 No.1, String Quartet in D, Op.18 No.3 etc, sometimes written in the style Op.18/3.

Composers like Bach, Handel, Mozart and Haydn were working musicians. Their works had a purpose – Bach's cantatas were written for specific days in the Church calendar; as Cantor of St Thomas's Church at Leipzig, it was expected of him – in effect, a part of his job description. Bach would also recycle elements of earlier compositions in later ones – perhaps because he was pushed for time and needed to finish a new work to a tight deadline. Many of the works by these and other composers were by no means intended for publication. Manuscript copies might circulate, but in a very limited manner.

In an interview³ about the mysterious number 69, Ligeti was asked: 'I'm intrigued by one title from 1951, your Grand Symphonie Militaire Op.69', to which Ligeti responded:

Oh that was a joke. The Opus number refers of course to the sexual position.

Other Numbering Schemes

For a few, notable composers, it became apparent that a new definitive catalogue of the sum total of their output was needed to sort out the hitherto confused situation. For these composers, their compositions are now known universally by their definitive catalogue numbers, rather than by opus numbers, if any of these existed in the first place.

Johann Sebastian Bach (1685-1750)

All of the individual items in Bach's huge output are now referred to by their BWV number –an abbreviation of Bach-Werke-Verzeichnis, or in English, Bach Works Index. So the St Matthew Passion, written in 1727, is BWV244 and the very famous D minor Toccata and Fugue, written not later than 1708, is BWV565. But hang on, BWV565 was written before BWV244? Yes, because the BWV index does not attempt to list the works in chronological order; instead it groups the works by their type. It starts with vocal works, then works for the organ, then harpsichord works etc. This is what is described as a Thematic Catalogue. The BWV index was published in 1950 by the German musicologist, Wolfgang Schmieder, and lists 1,127 works!

Wolfgang Amadeus Mozart (1756–1791)

The definitive catalogue of Mozart's compositions was generated by Ludwig von Köchel in 1862. Works are identified in the catalogue by their K number – the Symphony No. 35 (Haffner) is K.385. Unlike the Bach catalogue, Köchel's attempts to be chronological, but for a large part it is mainly educated guesswork. Since its original publication, there have been a number of revisions to the catalogue, to take account of new works discovered and of changes in attribution. The final work listed, K.626, is the unfinished Requiem in D minor.

Franz Schubert (1797-1828)

Schubert's compositions are most frequently referenced by their D numbers. This is how they are listed in the catalogue produced by Otto Deutsch and published in London in 1951. A new revised and enlarged ('D2') edition was published, in German only, in Kassel in 1978. Like Mozart's Köchel catalogue, the Deutsch index is chronological, and includes almost 1,000 entries.

These are far from being the only three composers whose works we identity by reference to a catalogue entry. Franz Joseph Haydn died in 1809, and yet there is still no complete catalogue of his compositions. A fair attempt was made in 1957 by Anthony van Hoboken, a Dutch musicologist, and many Haydn works are today often identified by Hoboken numbers (abbreviated as H or Hob). Like Schmieder's Bach offering, Hoboken's is a thematic catalogue, but it is now considered outdated and in need of a replacement to incorporate the latest in Haydn research made since 1957, a good deal of it by the Haydn specialist HC Robbins Landon.

So all in all, the opus number can be a bit of a mixed blessing, and may or may not tell us what we want to know. It was also not uncommon for a composer to start a work, lose interest and lay it aside for perhaps years, before returning to complete it. In cases like these, should the opus number reflect when the work was started, or when it was finished?

With their system of publication dates, editions and reprints, publishers of literary works win hands down over publishers of music.