To understand the difference between “which” and “that,” first you need to understand the difference between a restrictive element and a non-restrictive element, because the simple rule is to use “that” with a restrictive element and “which” with a non-restrictive element.After clearly explaining the grammar of that statement, Fogarty provides this Halloween-appropriate "Quick and Dirty Tip":
If you think of the Wicked Witch (Which) of the West from The Wizard of Oz, you know it’s okay to throw her out. You won’t change the meaning of the sentence without the which phrase. So, you can throw out the which (or witch) clause, commas and all.
If you can safely throw out the “which” and the meaning of the sentence doesn't change, then you know “which” is the right choice. If you try to throw out the clause and it does change the meaning of the sentence, then you know that the right choice is “that” instead of “which” because it's a restrictive clause. ...After providing some "advanced" advice, Fogarty concludes with this clear summary:
[T]he simplest rule is to choose the relative pronoun “that” when you can't get rid of the clause and the relative pronoun “which” when you can get rid of the clause. Remember that it's always safe to throw out the “whiches.”While I agree with and recommend Fogarty's advice, here's my advice from Garbl's Editorial Style Manual. It includes some related advice on using who and whom:
That is the defining, or restrictive, pronoun for essential clauses: The camera that is broken is in the shop (tells which one). Which is the nondefining, or nonrestrictive, pronoun for nonessential clauses: The camera, which is broken, is in the shop (adds a fact about the only camera in question).
In the examples above, note the correct use of commas: Which clauses are always set off with commas (or sometimes dashes or parentheses), and that clauses aren't. Essential that clauses cannot be cut without changing the meaning of a sentence. Don't set off an essential clause from the rest of a sentence with commas. Nonessential which clauses can be dropped without altering the meaning. Set off a nonessential clause with commas.
James J. Kilpatrick, The Writer's Art, 1984: "Rule of thumb: If the qualifying phrase is set off by commas, use which; if not, use that."
In addition, that is the preferred pronoun to introduce clauses that refer to an inanimate object: Greg remodeled the house that burned down Friday. Which is the only acceptable pronoun to introduce a nonessential clause that refers to an inanimate object: The house, which Greg remodeled, burned down Friday.
When an essential or nonessential clause refers to a human being or something with human qualities (such as a family), introduce it with who or whom. That -- but not which -- also may be used to refer to human beings, as well as inanimate objects. Don't use commas if the clause is essential to the meaning. Use them if it is not.
Theodore M. Bernstein, The Careful Writer, 1977: "Which normally refers to things, who to persons, and that to either persons or things."_________
Fogarty's article is featured today (Nov. 1) in my daily online paper, Garbl's Style: Write Choices--available at the Editorial Style tab above and by free email subscription.