Which vs. That

Q. Could you please explain to me the proper usage of “which” vs. “that”? 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 was 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 was 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 (without it, there’s no question as to what is the largest change in tax regulations in the past twenty years).

The basic rule: Use “which” plus commas to set off nonrestrictive (unnecessary) clauses; use “that” to introduce a restrictive (necessary) clause:

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.

If you remove “that’s less than an inch deep” from the first sentence, it becomes inaccurate. The clause is said to restrict the meaning of the sentence; therefore, “that” is correct.

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.

CMOS covers this issue in several places. For starters, see 6.22. Then take a look at the entry for “that; which” in the “Glossary of Problematic Words and Phrases” following paragraph 5.220.