The more you write, the more often you’ll run into the fork in the road called that versus which. That is, which do you use and in what instance? A new book in the Workshop’s library is Woe is I, one that bills itself as “the grammarphobe’s guide to English in plain English.” It’s true — and possessives are also a meaty chunk of the book — this book by Patricia T. O’Connor is written in plain English. So that’s why, which you might have guessed from the book’s scope, Woe is I delivers a ruling on That v. Which on page 2.
The sentence above uses which correctly because the clause containing which can be pulled out the sentence — and we still get the point of the sentence. Not all clauses are so conveniently set off by commas, though. The clause that is bereft of commas is the one that needs that. One best way of thinking of this is, “can I get away with which here, or do I take the default of that?” That makes “which” a fast friend of the comma. Meanwhile, “that” is often the best choice in every other circumstance. O’Connor leaves us with a little epigram to sort out the distinction.
Commas, which cut out the fat
Go with which, never with that.