Which vs. That
Q. Could you please explain to me the proper usage of “which” vs. “that”? CMOS does not explain it in detail, and I could really use a “hard-and-fast” rule to keep in mind regarding proper usage of these terms. Here is an example of the actual sentence currently in debate:
The Economic Growth and Tax Relief Reconciliation Act of 2001 which became law on June 7, 2001 is the largest change in tax regulations in over two decades.
I felt that in this instance “which” should be replaced with “that,” or that the phrase “which became law on June 7, 2001” should be set off in commas. A coworker disagreed, saying that “which” is correct because there is only one Economic Growth and Tax Relief Reconciliation Act of 2001, but that doesn’t seem right to me. Should we just have rewritten it to say “The Economic Growth and Tax Relief Reconciliation Act of 2001 passed on June 7, 2001 is the largest change in tax regulations in over two decades”?
HELP! Thank you.
A. First, the correct form for the sentence you cite:
The Economic Growth and Tax Relief Reconciliation Act of 2001, which became law on June 7, 2001, is the largest change in tax regulations in over two decades.
The phrase “which became law on June 7, 2001” is not necessary to the meaning of the sentence; remove it and the sentence still makes sense (for one thing, without it, there’s no question as to what is the largest change in tax regulations in the past twenty years). Your colleague is right to point out that it is important that there was only one such act of 2001, but when you use “which” for a nonrestrictive (unnecessary) clause, you must set it off with commas.
The basic rule: Use “which” plus commas to set off nonrestrictive clauses; use “that” to introduce a restrictive clause. If there had been two or more “Economic Growth and Tax Relief Reconciliation” acts in 2001, “that” would have been correct:
The Economic Growth and Tax Relief Reconciliation Act of 2001 that became law on June 7, 2001, is the largest change in tax regulations in over two decades. The one that became law on June 1 was not.
Note that a comma is required after 2001 regardless. This is a strict rule, but once you start to set something off (in this case, with the comma before 2001), you must finish setting it off so that it doesn’t look as if it belongs only to what comes after.
Here are some less complex examples that might illustrate the point more succinctly:
Pizza that’s less than an inch deep just isn’t Chicago style.
Pizza, which is a favorite among Chicagoans, can be either bad for you or good, depending on how much of it you eat.
Note that if you remove “that’s less than an inch deep” from the first sentence, it becomes inaccurate; i.e., it’s not true that “pizza just isn’t Chicago style.” The clause, then, is restrictive (necessary) to the sentence; therefore “that” is correct. You could even leave “that” out:
Pizza less than an inch deep just isn’t Chicago style.
If, however, you take out the clause “which is a favorite among Chicagoans” from the second sentence, it still makes sense: i.e., pizza can be either bad for you or good, and whether or not it is a favorite among Chicagoans does not “restrict” this meaning; therefore the clause is nonrestrictive and should be introduced by “which” and set off by commas.
Some people use “which” restrictively, which is more or less okay (and popular among writers of British English) as long as no commas are involved:
Pianos which have a fourth pedal to mute the strings are popular among apartment owners.
See paragraphs 5.58–63 in the fifteenth edition of The Chicago Manual of Style for more information, and, for a succinct statement of Chicago style when it comes to which vs. that, see the entry for “that; which” in the “Glossary of Troublesome Expressions” at paragraph 5.202.
