Looking for some grammar advice that is sure to put you in the Halloween spirit? Then look no further. And by the way, notice I used that, not which.

That’s because¬†that introduces a restrictive clause, a clause that is essential to the identity of the sentence element it modifies. In this instance, the clause “that is sure to put you in the Halloween spirit” is essential to the identity of the modified element, namely “some grammar advice.” Remove the that-clause and you’re left with “Looking for some grammar advice?” Maybe you are, or maybe you aren’t, but you certainly don’t have any idea what type of grammar advice I’m asking about.

A clause can also be nonrestrictive. Unlike restrictive clauses, nonrestrictive clauses provide supplement information. This means that they can be omitted without fundamentally affecting the identity of the sentence element they modify. Consider these two examples: 

One witch that always scared me as a child was the Wicked Witch of the West.

The Wizard of Oz, which features Margaret Hamilton as the Wicked Witch of the West, always scared me as a child.

In the first example, the clause “that always scared me as a child” is essential to the identity of the sentence element it modifies, “One witch.” (Which witch? The one that always scared me as a child.) In the second example, the clause “which features Margaret Hamilton as the Wicked Witch of the West” is not essential to the identity of modified element, “The Wizard of Oz.” The which-clause provides supplemental information and can be removed without fundamentally affecting the identity of the modified element. As the above examples show, commas are used around nonrestrictive clauses but not around restrictive clauses.

Want to learn more about restrictive and nonrestrictive clauses? Then be sure to check out this post. We’ve even got a quiz you can take to test your knowledge.

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Susan Doose

Susan Doose is an associate editor at the MLA. She received her PhD in German studies from Rutgers University, where her dissertation focused on the function of framing devices in German realist literature. Before coming to the MLA, she worked as a freelance copyeditor, translator, and German-language teacher.