Opus, Köchel, BWV...are they on the test?
You pick up a CD booklet or open a concert program, and there they are -- those pesky numbers! They come after some of the works listed, usually preceded by an abbreviation like Op., K., or BWV. What are they, why are they there, and do you have to know them? And why do you see them on New England Public Radio's on-line playlist, but rarely hear them on the radio? I'm glad you asked (ahem)!
First, a few definitions. Please don't be offended if you know this, or embarrassed if you don't. There are plenty of both kinds of listeners in every classical audience.
Opus (abbreviated Op.), the Latin for "work", was used (it rarely is now) by composers or their publishers to indicate the published order of their compositions. In other words, Beethoven's "Appassionata" Sonata, Op. 57, is from his 57th publication (some publications contain more than one work, such as Beethoven's three Piano Sonatas, Op. 2). Most of the other numbers refer to a musicologist, or sometimes several working together, who catalogued a composer's complete works, assigning each a number. In some cases, as in Ludwig Köchel's catalogue of Mozart's works (the source of the K. numbers), or Otto Erich Deutsch's catalogue of Schubert (where we get the D. numbers), the works are ordered chronologically. In other cases, such as the Bach Werke Verzeichnis ("Catalogue of Bach's Works", abbreviated BWV), the works are ordered by genre.
So that answers the "what", when it comes to the numbers in your program. How about the "why", as in "why on earth are they there"?
The short answer is, because they've always been there. Or at least they have since the late 19th century, when the concept of a "classical" repertoire by great dead composers came to dominate the concert scene. Its rise coincided with the development of musicology as a separate academic discipline, one dominated by Germans. And we know about the Germans and their propensity to put Alles in Ordnung ("everything in order"), as well as classical music's resistance to change. So, like it or not, we're stuck with 'em.
Not to say the numbers don't sometimes serve a purpose. Antonio Vivaldi, for instance, composed a few dozen concertos for the bassoon, several of them in the key of B-flat major (we'll get to keys some other time). How to tell them apart? With their RV numbers, or course. That abbreviation, by the way, stands either for "Ryom Verzeichnis" ("Ryom Catalogue", named for musicologist Peter Ryom), or "Répertoire des oeuvres d'Antonio Vivaldi".
But let's say that one of Vivaldi's Bassoon Concertos in B-flat major was on the classical show today, and I announced it as "RV 502" to distinguish it from 501 and 503. Would you care, or even remember what number I said ten seconds later? Unless you're a bassoonist (which I use as a term of endearment), probably not. So, I almost never do. Same goes with Beethoven's Symphony No. 5. You'd know it from that title alone, and hardly need to have "Opus 67" added to identify the piece. By contrast, Beethoven's earliest group of string quartets, are usually referred to as the "Opus 18 quartets". In common usage, that's their name. So, that's what we call them on the radio.
On the other hand, we make a point of including all the numbers, keys, and other pertinent information on our on-line playlists. There they can sit, not doing anybody any harm, ready to be cited if you want to, or ignored if you don't. You don't have to know them to enjoy the music, but if you want to know them, there they are.
P.S. If you write a comment on this post, please refer to NEPRCB101311 (i.e., New England Public Radio Classical Blog for 10-13-11). Otherwise, how could we possibly know which post you're referring to?