That or Which?

One of the trickier grammar rules is the difference between the words that and which. Both serve a similar purpose of clarifying information in a sentence, but the meaning of the sentence can change depending on which word you use. 

There is an easy rule to remember "that" vs. "which":

• “That” introduces essential information in a “restrictive clause.”

• “Which” introduces extra information in a “nonrestrictive clause.”

This mnemonic device will help you remember the rule:
"Which" has more letters than "that" and therefore brings extra information vs. only the essential information. 

“That” Example:

“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.

A comma is never needed with that in a restrictive clause.

“Which” Example:

“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.

A comma is required with which in a nonrestrictive clause.