the battle of that and which

(Sorry if this topic has been covered before.)
All right, grammarians: what’s the difference in usage between “that” and “which”?

I’ve been told that “that” begins information that identifies or specifies the noun that it follows, while “which” begins information that is unnecessary or incidental to the noun that it follows.

I guess that makes sense, but it’s hard to understand to the extent that it affects real usage. Can anyone comment, clarify, or otherwise enlighten me/us? Thanks.

Yes, that’s right.

  1. If it’s optional information, which can be set off in commas, use “which.”

  2. If it’s optional information, use “which.”

  3. If it’s information that is necessary to the sentence, use “that.”

  4. If it’s information, use “that.”

Note that sentence #4 is grammatically correct, but makes no sense.

More examples:

This is the dog that bit me.
This is a dog, <which is> a four-legged quadruped.

Definite nouns tend to require “that” clauses; indefinite nouns are more likely to take “which.” When referring to people, you can use “who” for either case, which is one reason why the whole thing is very confusing.

Dr. Drake gave you the classic restrictive versus non-restrictive clause argument. You won’t be criticized for using this, it’s the only rule out there on this, but know that plenty of writers deviate from it without coming off as unclear or uneducated. It seems the best argument for replacing a usage of the restrictive “which” with “that” is that people may confuse “which” with “which,” (i.e., restrictive usage versus non-restrictive). For that I feel like shouting “you won’t understand it only if you don’t understand English!” If you express yourself clearly, you can use either really. Try this brief article.

Either way that goes, not only “can” you refer to people using who, the rule has long been you should refer to them as such. Don’t use a word used for objects to refer to people.

:smack: Hell… I meant to type “which” and “, which”.

“Which” is required, by custom of usage, for non-restrictive clauses. It will sometimes appear used restrictively, a circumstance in which prescriptivists prefer “that” when possible. (Notice that “which” is the preferred object of a preposition.) Also note that “that which” is the standard alternative to the noun-clause relative pronoun “what”: e.g., “What goes round, comes around” is equivalent to “That which you do, returns to hurt you.”

It would be nice if there were one single rule, which would apply in all circumstances. But the instances in which “which” is used and those that “that” is used don’t fall neatly into simple groups that can be easily defined. What one needs to do is to develop an ear for the varying usages.