system
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human
ELI5: How does the naming system behind classical music pieces work?

symphony 4 movement 6 concerto 9 op 16 no 32 in g major. 

i just made that up obviously, but how does one make sense of it?
gpt
Good question. "G major" and the like refer to the key of the piece (that is, the scale on which the piece is based). 

Opus number indicates the order in which a particular composer's pieces were published. Since Mendelssohn's A minor string quartet was his 13th published piece, it's called op. 13.

Sometimes composers publish several pieces at the same time in one big collection. In that case, the collection as a whole gets an opus number, and each work in that collection gets a number of its own. For example, Beethoven's F major string quartet was the first in the collection of quartets known as Opus 18, so we label it op. 18 no. 1.

A movement is a discrete section of a piece that usually stays in one tempo. For example, the "Allegro con brio" section of Beethoven's op. 18 no. 1 quartet is the first movement of that quartet.

Symphonies and concertos are two different types of pieces. A symphony is a long piece for orchestra with usually four movements (fast - slow - triple meter - fast). A concerto is a piece for orchestra *plus* a flashy, virtuosic solo instrument part, like a violin or piano. A concerto usually has three movements (fast - slow - fast).

The number you sometimes see next to the word "symphony" (or "sonata," or "concerto"...) just refers to how many symphonies (or sonatas, or concertos...) that composer wrote before composing the one in question. So Beethoven's Symphony no. 7 is the seventh symphony Beethoven wrote.

One more complicating factor: scholars have created catalog numbers for the works of some of the great composers. These are like Dewey decimal numbers at the library in that the number can tell you the genre of a particular piece. For example, Bach's first sonata for solo violin is labeled BWV 1001. The BWV stands for Bach-Werke-Verzeichnis (Bach Works Catalog), and the fact that it starts with 100- means that it's a solo instrumental piece.

As you can see, there's great opportunity for redundancy in these naming systems, which is why you'll rarely see a piece labeled with all the information you provided in your example. For instance, if you say "the second movement of Beethoven's fourth piano concerto," everyone will know what you mean.