We now come to an issue that has provoked swearing matches in recent months: how to choose between that and which as relative pronouns. Consider:
Republicans oppose new taxes that are unnecessary. (Some taxes might be necessary.)
Republicans oppose new taxes, which are unnecessary. (None, in their view, would ever be necessary.)
Republicans oppose new taxes which are unnecessary. (Ambiguous.)
Most American editors don’t like the third version because of its palpable ambiguity. They insist on choosing between a commaless-that and a comma-plus-which. A commaless-that clause narrows (restricts) the meaning; a comma-plus-which doesn’t narrow (it’s called nonrestrictive). The choice of the relative pronoun reinforces the semantic difference conveyed by the presence or absence of a comma.
So that’s that.
Except it’s not. Though it was born in Britain, this useful distinction has really taken hold only in American English. Hence many British authorities (e.g., Gooden, Greenbaum, Trask) say that it is permissible to use a commaless-which to introduce a restrictive clause, as here: I saw the speeding car which ran over your cat. In that sentence, most American editors would either change which to that or put a comma before which — after discerning the true meaning.
Some American commentators (e.g., Lynch) minimize the importance of the American rule while still reciting it: “It boils down to this: if you can tell which thing is being discussed without the which or that clause, use which; if you can’t, use that” (Lynch, p. 218).
Other commentators cite what they consider exceptional cases: We want to assign only that book which will be most helpful. But such sentences can always be improved by other means: We want to assign only the book that will be most helpful.
The only real exception occurs with by which, for which, in which, etc.: We’ve consulted all the cases in which Justice Kagan has recused herself.
My advice: heed the that/which distinction. Use many more thats than whiches. Try not to be distracted too much when reading current British writing (in which commaless-whiches are rampant). And know that many British sources (Ayto, Times Guide) do hew to this nuance, which Americans have elevated to the status of a rule.
Which stalwart American sources champion the rule? The Chicago Manual of Style, Bernstein, Follett, Garner (ahem), and Words into Type, among others.
This is the house that (not which) Fowler built.
John Ayto, The Oxford Essential Guide to the English Language 58-59, 289 (1995).
Theodore M. Bernstein, The Careful Writer: A Modern Guide to English Usage 443-46 (1984).
The Chicago Manual of Style § 5.220, at 298, § 6.22, at 313-14 (16th ed. 2010).
Wilson Follett, Modern American Usage: A Guide 321-24 (1966).
Garner’s Modern American Usage 806-07 (3d ed. 2009).
Garner, The Winning Brief 252-54 (2d ed. 2004).
Philip Gooden, The Guide to Better English 156 (2d ed. 2001).
Sidney Greenbaum & Janet Whitcut, Longman Guide to English Usage 705 (1988).
Jack Lynch, The English Language: A User’s Guide 218-19 (2008).
The Times Guide to English Style and Usage 144 (Tim Austin ed., 1998).
R.L. Trask, Mind the Gaffe: The Penguin Guide to Common Errors in English 281 (2001).
Words into Type 190, 378-79 (3d ed. 1974).
Bryan A. Garner, President of LawProse Inc., is the most prolific CLE presenter in the U.S., having trained more than 150,000 lawyers and judges. His book — most prominently Black’s Law Dictionary and Garner’s Modern American Usage — have been cited as authority by every state and federal appellate court, including the highest. For more about him, go to www.lawprose.org. To follow him on Twitter: @bryanagarner.