Dear Master’s student,
Here in southern California, spring has definitely sprung! To celebrate the return of warm, beautiful weather, we’ll discuss a topic that I don’t consider crucial to strong writing, but one about which I still get plenty of questions: that, which, and commas.
First, let’s talk about “restrictive” vs. “non-restrictive” clauses. A restrictive clause is one that provides information that is necessary to identify someone or something. A non-restrictive clause provides “extra” information, i.e. info that a reader would not need to identify someone or something. Commas are placed before and after a non-restrictive clause. You’re likely a little confused now, so let’s use examine an example for clarity.
Restrictive: My sister who normally avoids spicy food wants Thai food for dinner.
Non-restrictive: My sister, who normally avoids spicy food, wants Thai food for dinner.
Do these two examples have different meanings? Yes, slightly. In the restrictive example, we know the clause “who normally avoids spicy food” is necessary to identify which sister wants Thai food. This indicates the speaker must have at least one other sister, because without this spicy info, we wouldn’t know whom the speaker meant.
Conversely, the non-restrictive example, which you can identify by the commas around the spicy clause, is not necessary to figure out who wants Thai food. From this, you can infer that the speaker has only one sister (because “sister” is sufficient by itself to identify who wants Thai food).
Note the use of commas in the first sentence of this paragraph: “the non-restrictive example, which you can identify by the commas around the spicy clause, is not…” Because there is only one non-restrictive example, “which you can identify…” is extra information—not necessary—so there are commas around that clause.
Lastly, “that” should be used with restrictive clauses. “Which” should be used to introduce non-restrictive examples.
The country that produces the most olive oil is located in Europe.
Spain, which produces more olive oil than any other country, is located in Europe.
In the first sentence “the country” isn’t enough information to identify the specific nation in question. So “that produces the most olive oil” is necessary (or “restrictive”); thus “that” is used and no commas go around the clause.
In the second sentence, “Spain” is sufficient to identify the country being discussed. That means the additional information is unnecessary (or “non-restrictive”), and “which” with commas should be used.