People often ask me whether which and that are interchangeable.
These words serve multiple functions. Which may open a question, as in, “Which one do you like best?” Few people doubt this usage. And that can point to a particular item, as in, “Can I have that one?”
However, questions pop up when which and that are used as relative pronouns. In this role, they separate the independent, central part of a sentence, from a dependent or subordinate clause. Remember that a clause is a set of words that contains both a subject and a predicate (a noun and a verb).
Relationships — and Relating Words — Are Important
The difference between which and that hinges on the nature of the clause they introduce.
Which introduces a clause that is not essential to the meaning of the sentence. We always sandwich the which clause between two commas.
That introduces a clause that is essential to the meaning of the sentence.
A clause that is not fundamental to the meaning of the sentence is called a nonrestrictive clause, meaning that it does not restrict the meaning of the sentence.
Notice in the above sentence that if you removed that is not fundamental to the meaning . . . you would end up with, A clause is called a nonrestrictive clause. Clearly, this sentence lacks guts. That’s how we know that the clause that is not fundamental . . .is essential to the sentence.
Here are other examples of nonrestrictive clauses:
By that time, the IRS, which had been after him for years, had finally caught up with him.
You could omit which had been after him for years and still end up with the core sentence,
By that time, the IRS had finally caught up with him.
A clause that is essential to the topic of the sentence is called a restrictive clause. We introduce these clauses with that. Consider this sentence:
The real problem was that his advisers gave him an unrealistic view of the situation.
If you take away that his advisers gave him an unrealistic view of the situation, you take away the heart of the sentence.
While following the rules about restrictive and non-restrictive usage is preferable, your ear for the language should be your ultimate guide. If which flows better in the sentence, use it.
Which is Which?
So, which word is the right one to use? That is a good question! Use which to introduce non-essential information; enclose it between commas. Use that for necessary information. And that is that.
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