There's usually a comma before "which" when it introduces a relative phrase with its own verb, etc.

However, even "that" can be preceded by a comma, depending on the sentence structure. :-)

Wenn ich mich recht erinnere, bekommt which ein Komma, wenn es einen non-defining relative clause einleitet (der also weggelassen werden kann), aber keins, wenn der Nebensatz defining ist.

Bsp.: The bar, which had opened only a year ago, was packed to the brim. non-defining, der Satz geht auch ohne Zusatz.

The answer which I got didn't make me happy. defining, Zusatz ist essentiell.

Are you ready? When "which" is used before a relative clause, you have to distinguish between a restrictive clause and a non-restrictive clause.
Sorry!

A restrictive clause is a clause that is essential to the meaning of the sentence. For example: (a) "The book which I read is well written." (b) "The only case of which I have direct knowledge occurred in 1972."

A non-restrctive clause is a clause that is not essential to the meaning of the sentence. For example: (a) "That book, which incidentally I just finished reading, is well written." (b) "The only case, of which I actually happen to have direct knowledge, occurred in 1972."

(This subject may invite intervention by prescriptionists who will say that the use of "which" is wrong before a restrictive clause; and that "that" must be used. Ignore them!)

There is a comma before "which", which is just a tiny word in English, if the phrase which starts with "which" is gibberish. The answers which I got made me happy :-) The answers, which I appreciate a lot, are almost always excellent. :-)

Thank you all.

tilmanb, you actually provided the best explanation yourself just now. What a sweet summary.

Actually, Martin provides the clearest summary of this question. But it must be added that nowadays, in most cases, "which" is used to introduce a nonrestrictive clause and "that" is used to introduce a restrictive clause.

To use Martin's examples:

"The book that I read is well written." (Not "The book which I read.")

but

"That book, which I just finished, is well written."

I once learned that, at least in formal language, a restictive clause should always be started with 'that', and a non-restrictive one with 'which'. Is this true?

If so, it would render the orginal claim (comma before 'which' not before 'that') indirectly true.

The first book I grabbed, 'Ready for First Certificate', McMillian 2003, still gives 'which' for non-defining and 'which' AND 'that' for defining clauses but then adds 'particularly in spoken English' (referring to 'that'). So I'd say Yes-ish.

This subject is one of a number that are perennial here in LEO. There must be several dozen threads on it already, if anyone cares to look them up. No doubt all would shed further light.

I think part of the issue has to do with an AE / BE difference. I learned it exactly as Martin gives it.

(And Bob C.'s point is correct.)

@Gibson, Martin et al.: I also learned that comma insertion is dependent on the difference between restrictive and non-restrictive clauses with "which" being acceptable for both.

Do any of you use Microsoft Word? Does Word flag the use of "which" in restrictive clauses as grammatically questionable/wrong? My current version (2003, XP) doesn't, but the version on my school computer (97) does. Of course, the program often makes totally spurious and incorrect suggestions based on who-knows-what sort of logarithm.

@ Bob C

'fraid I have to disagree with you again. Just because Microsoft Word's grammar-check keeps saying: use "that" rather than "which", doesn't mean one should.

A comma precedes "which" when the latter introduces a non-defining (also called non-restrictive) relative clause, the kind where also you can't replace "which" by "that". And of course if it's not the subject, you can leave it out altogether in

@escoville #12 - I agree wholeheartedly. Just to avoid confusion: my comment in #11 was intended as a criticism of MS Word.

::rant mode one:: One of the things I hate most about the program is that it tries to cram everyone into a restrictive mold that stifles creative excellence in the name of "standardization" (read, conforming mediocrity). Even formatting becomes a problem at times. I know what I want the page to look like, but the program "thinks" its pre-programmed "look" should override my preferences. ::rant mode off::

okay, take back the e - ::rant mode on::

I'm going by Fowler (the new as well as the old), Bernstein, and the Chicago Manual of Style. I could provide citations if it were of interest to anyone.

As we've said before, everyone is free to write as they please.

Robert and Escoville: I don't mean to have an argument with you, and I shouldn't take the tone that I sometimes adopt (as in #15).

I mean it when say that everyone should write the way they find comfortable and correct.

The thing is that the dictionaries and manuals of style do set the accepted standards for formal writing for publishing, journalism, and academic papers. No one has to conform, but when people ask us what is correct, we have an obligation to distinguish between our personal preferences and those of the standard reference works--to which they obviously often do not have access.

If they are translating for customers, clients, employers, professors, teachers, or exams, they need to know what comes from reputable sources and what represents our personal preferences.

@Bob: Thanks for the comments. I have to admit that my writing (and, to a degree, even my speech) is still influenced by having grown up reading the King James Version of the Bible.

My rant against MS Word is still valid. Some of the suggestions are truly absurd:
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty and the pursuit of happiness."
MSWord marks the words in boldface as grammatically unacceptable and suggests the following: 1) "rights that" or 2) "rights, which". The program governing the grammar checker obviously is not sufficiently sophisticated to deal with this absolutely acceptable construction.

"The thing that makes a creative person is to be creative and that is all there is to it." (Edward Albee) MSWord marks this: "Fragment (consider revising)" A fragment?!

Robert, I agree with you about MS Word's editing capabilities. I scarcely bother with it.

I suppose it can be useful to those who do a high volume of writing or translating--if it is used wisely.