The

Why ask why? As with many things in English, it has its own special rule:

http://www.uta.fi/FAST/BIE/MAC/PP1A/pp1a-lec17.html

But I suspect it is similar to when we speak of categories or generic objects:

The fruit bat is a curious animal.
The comma is often abused.
The piano has 88 keys.
(compare to: "A piano has 88 keys.")
In the US, we drive in the right lane.

Jun-Dai you did not get the point mate.

What, pray tell, is your point then? My point is that it isn't following any particular rule (i.e., it has its own special rule), but that there are some very similar uses of the definite article.

The idea behind using the definite article in the case of playing a musical instrument, is that you habitually play one particular kind of instrument out of many, or that you are somehow describing the instrument in its abstract form. What is strange is that it holds true for instruments and dances, but not for sports (I play the tennis?), cooking (I cook the pasta?), or other things (I read the romance novel? I drive the car? I program the computer?).

It may be useful to compare the common practice in certain Romance languages, where the article is used for pretty nearly all nouns. I've found French particularly illustrative, in which you may have la/le or un/une, corresponding respectively to "the" and "a" in English. One may think of a case like French as being "regular," following a standard usage rule, while the same case in English might be thought of as "irregular."

Maybe someone much more familiar with the use of the article in French, Italian, or Spanish could provide us with some typical examples.

Jun-Dai had the right idea (the definite article being used to describe the instrument in the abstract and/or generically) and the website he linked to had a good point as well ("the" being used when talking about something that is common knowledge or that should be known by the listener), but speedwell capped it off. The definite article is used quite a lot in English, but quite a bit more in French and other Romance languages, so far as I've seen. That's probably where English got it from. However, if you're uncomfortable using the definite article to describe, in your example music, then it can usually be dropped and the phrase will still make grammatical sense. "I play the guitar." makes just as much sense as "I play guitar." although sometimes the noun must be pluralized (taking an example from Jun-Dai, "The fruit bat is a curious animal." would be "Fruit bats are curious animals."

No article is used in that case. "I play guitar", that's it. Get an English grammar book, something like "A Practical English Grammar" or "Three words" by Claire, or something similar.

So, passing by, I suppose that in thirty years of piano study, including three years of formal university and ten years of giving lessons, I haven't managed to figure out yet that "I play piano," as opposed to "I play THE piano."

Take your grammar book and go sit on a tack...

The Beatles, being introduced to a TV show, would say, "My name is John Lennon, and I play a guitar." This always sounded to me like they only ever learned to play one guitar each, but I've heard it elsewhere. I usually say, "I play guitar," or but I hear and use, "I play the guitar."

(Ringo, on the other hand, would say, "I play the drums," but he did have more than one of them.)

I think "I play the guitar" gives the feeling of some mastery or expertise, or at least effort expended, while "I play guitar" is less charged with extra meaning.