Today, I'm going to tackle the proper use of which vs. that . Before we get started, commit this to memory: Which needs a comma(s). That does not. Ex: My first car, which is in the driveway, is red . Ex: The dog that stole my hotdog ran away . Next, you need to understand the difference between a restrictive clause and a nonrestrictive clause. A restrictive clause is essential to the meaning of the sentence. If you took it out, the sentence wouldn't make sense or its meaning would change. A nonrestrictive clause is one that adds additional information, but could be left out without losing anything vital. *Nonrestrictive clauses are set off with commas. Restrictive ones are not. Let's apply this to our examples... Ex: My first car, which is in the driveway , is red . Ex: The dog that stole my hotdog ran away . In the first example, 'which is in the driveway' is additional information. We already know which car—my first car. In the second example, the clause must