I dont get it...

I can't explain it better than this....

http://www.classical.net/music/composer/opus.html

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Thanks for the link Sugarbrie

Man, that stuff is way further in depth than i expected.

Its kinda like reading the periodic table of elements.

HAHAHA

You have to keep the stuff straight some how.

Didn't the Germans use these codes in WWII?

In addition to the excellent explanation offered in Sugarbrie's post,let me mention something else.

Many of us find these numbers hard to keep straight and classical publishers(especially pre-electronic aged) sometimes named compositions for the convenience of their customers who ordered music from them.

Here are just a few:

--Mozart, Jupiter Symphony,Emporor Concerto
--Hayden,Suprise,Drumroll,and Bear Symphonies
--Beethoven,Moonlight Sonata,Pastoral Symphony

I could go on but you get the idea.

That a piece of music had a publisher assigned nickname meant that it was popular among buyers and does not guarantee you will like it, but that many others did,whatever that is worth.

Mozart did not write the EMPEROR Concerto; it is Beethoven's 5th piano concerto.

Do your math!
It's easier to integrate and take derivatives as well:-)

My mistake. I meant to say the "Coronation" piano concerto. Thanks for pointing it out.