Sorry to disappoint.
The Classical Era
Recap and audio links:
Note to self: "recaps" shouldn't be longer than what they recap...
Also, a note on the weird letters and numbers in the titles of pieces...
Composers wrote lots of pieces of music, which makes it hard to identify which piece is meant (particularly when translating between languages, and particularly when many 'names' are taken as descriptions that are partly subjective). This was particularly a problem in the Classical, when composers were insanely productive, but mostly in a small number of genres. Haydn has at least 104 symphonies, for example. "Symphony in A" isn't enough to identify them.
One way to deal with that is to number the symphonies. But early composers tended not to number their symphonies themselves, and as symphonies reached different publishers and concert halls in different orders, there was no definitive, objective order to number them in. Composers didn't set out to write, nor did impressarios advertise, their "fourteenth symphony" - they just advertised "so-and-so's latest symphony!" Later scholars can number the works, but sometimes doing so conflicts with an existing established numbering. Haydn's symphonies, for instance, are numbered, but the traditional numbering is not chronological - because, as the privately-employed composer grew more famous, he published "new" symphonies that were actually things he'd written years or decades earlier but that hadn't previously been performed publically. Some composers/publishers even, duplicitously, published the same work twice with different names or numbers. Besides, numbering within a genre runs into the problem of defining genres - is it a sinfonia, or a symphony, or sinfonia concertante, or a concerto grosso, or a concertante symphony, or...?
So, scholars of particular composers went back and constructed would-be comprehensive catalogues of exactly what was written, attaching a number to each work. Most of these catalogues are chronological, or attempt to be, but some instead categorise by genre. Unfortunately, different scholars came up with different catalogues - a new piece was discovered, or an old piece realised to be misattributed, or, in chronological catalogues, a piece realised to be associated with the wrong composition dates, and so forth. [originally, these catalogues were drawn up ad hoc by biographers as appendices; later, writers had to include not only lists of works but also complicated conversion tables to explain how their list corresponded to all the lists in all the other biographies]. To make things slightly clearly, catalogue numbers are therefore accompanied by an abbreviation to indicate the catalogue. Fortunately, for most composers a particular catalogue has achieved predominance.
So, for instance, there are half a dozen modern ways of counting Bach's works (and thus working out which "Toccata in D" or "Prelude in E" somebody means), but in practice one virtually only ever sees things listed with a "BWV number" (Bach-Werke-Verzeichnis). Similarly, Mozart's works are almost always identified by their K-number (for Koechel). More obscurely, for example, William Friedemann Bach gets Fk (Falk) numbers, while CPE Bach gets Wq (Wotquenne) numbers. Vivaldi gets RV numbers, for "Ryom-Verzeichnis".
This tends to only be an issue in the Baroque and the Classical. Before that, nobody cares enough, and not enough is really known, so things tend to be identified by what book they were published in originally (so far as we know). Kind of like referring to pop songs bytheir name and what album they first appeared on. During the Classical, publishers began attaching "opus numbers" - their own numbering of published works. Opus numbers, written "Op.", have two potential problems - first, not everything got formally published; and, second, things aren't always published in a logical order, or may even be published with two numbers. However, as time went on, opus numbers got to be a more and more reliable way of identifying works, so they're the default for Romantic, and many modern composers. [later, Op numbers stopped being tied strictly to publishing schedules, and composers assigned their own Op. numbers, effectively as part of the name of the piece]. Beethoven straddles that line, so most of his works get an opus number, but there are enough that he never published that there are also many works with "WoO" numbers instead (Werke ohne Opuszahl), referring to a scholarly catalogue of unpublished works (Fuer Elise is WoO 59 (and occasionally Bia 515)). Schubert didn't publish much while alive, so he gets D numbers. Dvorak didn't publish all his stuff, but more particularly his publishers really dicked everyone around with what numbers they gave things, so his works are often known by B numbers. Debussy was all hipster and didn't like labels man so he refused to give his works opus numbers, so now everything has to be labelled with L numbers. Etc. But for most composers, it's "Op."
[notably, opus numbers don't always refer to a single piece, when several pieces were published in a single omnibus edition, in which case the pieces are known by "opus" and then "number".
Thus, one sonata by Beethoven may interchangeably be known as "the Moonlight" (nicknames were a common early way of being clear which piece you were talking about), the "Quasi una fantasia" (the subtitle he gave the piece, but confusingly also to another piece, so this is ambiguous, but everyone knows which one you mean), "Piano Sonata in C# minor" (or C#-moll), "Piano Sonata no. 14", "Opus 27, number 2", or any combination of the above. Probably ideally "Piano Sonata no. 14 in C#-minor "Quasi una fantasia", Op. 27 no. 2 ("Moonlight")". ]
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!