When to use the relative pronouns that and which may rank as the No. 1 stickler in English grammar, regularly stumping both the professional and nonprofessional writer. Simply stated, use that to introduce essential clauses, and use which to introduce nonessential clauses.
But what are essential and nonessential clauses?
Here’s how the Associated Press Style Guide defines them: “The essential clause cannot be eliminated without changing the meaning of the sentence. … The nonessential clause can be eliminated without altering the basic meaning of the sentence.”
Let’s look at some examples of how that and which work with the two types of clauses.
Her detailed systems integration plan, which can be implemented under various conditions, is ideal for this situation.
In this case, the clause “can be implemented under various conditions” is not essential to the sentence. Remove the clause, and the sentence will still make sense. Therefore, which is correct.
Contrast this with the following example:
She developed the detailed systems integration plan that can be implemented under various conditions.