That vs Which
I cannot think of two more maddening words (or relative pronouns, if you will) in the English language than « that » or « which ». I always fear being asked to explain their correct usage. The truth is, no matter how many times I memorize the rules for “that” and “which,” within six months I go back to breaking them and using “that” and “which” interchangeably. And why? Because that is pretty much what the vast majority of people do with “that” or “which.” We don’t typically distinguish between them in the U.S. and thus interchangeability is increasingly becoming the rule and really, most people don’t give a fig that they are breaking grammar history.
Indeed, I would argue that as many as 95% of Americans use “that” and “which” interchangeably in both formal and informal spoken and written English. Only a handful of people (among them “purists”, “grammarians” and “linguists”) readily know the traditional rules for “that” and “which”; fewer still care to enforce their usage. And so, when English language learners ask for an explanation I am tempted to say to forget the rules and just use either one according to your preference since that is what most people do.
This may be a cop out, however. Because there are clearly times when “that” is correct and other times when “which” is correct. To conflate the two is to, in some instances, commit linguistic heresy; and in some extreme cases, it could even lead to liability if a contract drafter uses a non-restrictive pronoun (which) when he or she meant to be restrictive (that) or vice versa.
So, what are the rules?
After an exhaustive troll online to get everyone’s take on this, I think I will explain the rules as follows:
Use “that” when a descriptive or explanatory clause (in this case a “restrictive relative clause”) is meant to restrict or exclude other possibilities. In other words, use “that” when the clause is essential to the meaning, understanding and intentions of the sentence.
For example: “The parties agree to use crude oil that is commonly used in the industry.”
If this sentence appeared in a contract, it is clear that the drafter is doing a bunch of important things. First, he or she is excluding any and all other types of crude oil except for the type “that is commonly used in the industry.” Second, the drafter is specifying the exact “agreement” between the parties. The agreement is not just to “use crude oil.” It is more specific. It is describing exactly what type of crude oil the parties intended to be used. It is telling the reader that only the crude oil “that is commonly used in the industry” was agreed to. Thus if one party used a different type of crude oil, we could be looking at a breach of contract situation here.
By the way, the sentence could have stopped at “The parties agree to use crude oil.” If it had, but the client had clearly wanted to specify what type of crude oil he or she meant, the lawyer could have possibly gotten into trouble with the client because the contract left out an essential clause. Essential clauses start with “that.” Essential clauses are also “restrictive” clauses and they begin with the restrictive, relative pronoun, “that.”
As for the non-restrictive relative pronoun “which”: It is more general and includes a larger pool of possibilities. That is, “which” introduces a clause that is not necessarily essential to the meaning of the sentence.
So for example: “The parties agree to use crude oil, which is commonly used in the industry.”
Even though this sentence seems identical to the one above, it is not identical. It is infinitely more nuanced in fact. In this sentence, the parties are agreeing simply to use “crude oil.” This oil may or may not be commonly used in the industry. The only agreement is to use crude oil. Industry standards are totally irrelevant to the agreement here and in this sentence this clause serves only as a parenthetical statement by the writer. In fact, if you chopped this sentence off and simply said, “the parties agree to use crude oil,” you would not be changing the meaning or intentions of the parties, even though at the end of the sentence, the writer seems to have had an extra burst of creativity and wanted to say, “oh, and by the way, crude oil is commonly used in the industry so of course that is what we will use.” Yes, but nobody agreed to be bound by industry standards in this scenario!
Thus, the clause “which is commonly used in the industry” is non-essential to the sentence and is not restrictive in any way, only adding non-essential information.
BTW, one would typically separate a non-essential clause with a comma. So, “which” is usually preceded by a comma whereas “that” is hardly ever preceded by a comma since one does not separate an essential clause from the main clause in a sentence.