Coincidentally, three clients requested clarification today about using “that” and “which” in a sentence. There is an easy rule to remember. “That” introduces essential information in a “restrictive clause.” “Which” introduces extra information in a “nonrestrictive clause.”
- “Business writing expertise is the skill that is most valued.” The clause “that is most valued” is essential to the meaning of the sentence, so the correct word is “that.” You cannot remove the “that” clause without changing the meaning of the sentence.
- “We should all pay attention to David’s new product idea, which is likely to triple sales next year.” The second clause provides extra information, and it is not essential to the first clause. Therefore, “which” is correct.