Free Grammar Help—Restrictive Clauses— That or Which?
That or which? This is one of those "rules" that's not reall a rule. H.W. Fowler, a guru of grammar from the early 20th century promoted the idea of this differentiation. I like it, and I follow it, but in fact, you can do what you like. The grammar tools of MicroSoft Word follow this rule. If you see a green line under a clause, this may be the reason.
You use that with restrictive clauses and which with non-restrictive clauses. What, you say? What the heck is a restrictive clause? Restrictive clauses are ones that are essential to the meaning of the sentence. I.E. they "restrict" the meaning of the sentence. Here's a use of "that" with a restrictive clause: "This is the web page that describes the use of that or which." We see that the words "that describes the use of that or which" tells us exactly which web page we are talking about. It restricts the reference to a single web page. More information on Fowler's opinion.
When we use "which" we are just supplying extra information. It is not necessary for the meaning of the sentence. "The web page, which explains that or which, was uploaded in August." In this case, the extra information about the content of the web page was not essential to the meaning of the sentence, which was about the date of uploading. Notice that the non-essential information (the non-restrictive clause) is set off by commas. It acts just like an appositive.
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When you use the grammar checker in MicroSoft Word, it always prompts you to choose "that or which" when it sees these words. It likes to see a comma before which, expecting a non-restrictive clause. You need to be sure about what kind of meaning you are implying when you choose whether or not to include a comma. Here are some more examples of restrictive and non-restrictive clauses.
The woman that I met online is stalking me. (Important to know which woman.)
The trip that I made to Mexico was a success. (Tells us which trip.)
The dog that ate my homework belongs to my sister. (Tells us which dog.)
My brother who just returned from Mexico has a sunburn. (Tells us which brother.)
The party, which was in the church hall, was loud. (The location is not essential to the meaning of the sentence.)
My brother, who just returned from Mexico, has a sunburn. (Ha! fooled you, non-restrictive clauses don't always have to begin with "which.")
The sandwich, which the witch wished was Swiss cheese, was eaten by rich kids on a picnic. (OK, now I'm being silly, so time to end this page.)
What's important to note is that a non-restrictive clause needs a comma. The part that is added on doesn't change the meaning of the sentence (it doesn't restrict it). It just adds more information.