The Eternal Question: Which or That?

Students are constantly asking me this question, which refers to adjective clauses*. My dirty little secret is that I didn't know the difference or which to use until I was in my late 20s and in graduate school. Finally, in grammar class one day, my professor made it crystal clear with a very simple explanation. Hopefully I can explain it as well as he did. In order to understand the main question, we need to go back to some basic questions.

1. What is an adjective clause?

Adjectives are words that describe nouns. Most commonly, you put them in front of the noun that is being described, like:

• the noisy baby
• the fluffy dog
• a gorgeous wife

(And I am proud to say I have all three of these in my family.)

A clause is a group of words that has a subject and a verb and can stand alone as a sentence. For example:

• The baby loves to destroy the living room.
• The dog drives me crazy sometimes.
• My wife is a wonderful mother.

If I want to use these clauses to describe the nouns from above, I would connect them as adjective clauses, which come after the noun. If I were to describe my family, this is how I would do it with adjective clauses.

• I have a noisy baby who loves to destroy the living room.
• I also have a fluffy dog that drives me crazy sometimes.
• I am married to a gorgeous woman who is also a wonderful mother.

(I consider myself the luckiest man on the planet. Who could ask for anything more?)

2. How do you connect adjective clauses to nouns?

This is done with a small group of words called relative pronouns, which are:

• who / which / that
• where / when / why

For this entry, we will focus only on which and that. I will write about where and when at a later time. Relative pronouns, and the rest of the adjective clause, must come directly after the nouns that they modify**.

Adjective clauses can be divided into two types: defining and non-defining clauses. This is the key to understanding which and that and whether you need commas or not.

3. What are defining and non-defining clauses?

A defining clause is a clause that gives necessary information. Without the information in the clause, the reader cannot understand the sentence. Important information is missing.

• The book is very interesting. (What book?)
• The puppy is really cute. (Which puppy?)
• Kai paid for his insurance with the money. (What money?)

The information that you need to answer the questions is in the adjective clause. (Notice that the noun usually has the. It can also be preceded by a or be a plural noun with neither a nor the.)

• The book that I am reading right now is very interesting.
• The puppy that my friends just adopted is really cute.
• Kai paid for his car insurance with the money that he earned from his summer job.

In all of these cases, I used the relative pronoun that and there are no commas.

A non-defining clause is unnecessary. You still have a complete, understandable main clause without the adjective clause.

• Call of the Wild is one of my all-time favorite books.
• My friend's new puppy is really cute.
• Kai paid for his car insurance with his own money.

However, if you want to add some extra information that may or may not be interesting to the reader, you can add it with a non-defining clause.

• Call of the Wild, which I just reread recently, is one of my all-time favorite books.
• My friend's new puppy, which they rescued from the Humane Society, is really cute.
• Kai paid for his car insurance with his own money, which he earned by working at McDonald's.

Each of these non-defining clauses uses the relative pronoun which and has a comma both at the beginning of the clause and at the end of the clause. If you have one comma, you need the second (unless it's the end of the sentence, in which case, just use a period.)

To summarize:

When do you use that? Does that need commas?

• That is used with defining clauses.
• That is used to connect necessary information that the reader MUST have.
• You NEVER use commas with that.
• Nouns that are followed by that usually have the or a in front.

When do you use which? Does which need commas?

• Which is used with non-defining clauses.
• Which is used to connect extra, bonus information that may or may not be interesting to the reader.
• You ALWAYS use commas with which at the BEGINNING and END of the clause.
• Capitalized nouns are almost always followed by which.
• Nouns that are followed by which usually have my or this or a possessive noun in front.

I hope this explanation helps. If anything is unclear or you need more explanation, please ask. Nothing is ever perfect the first time no matter how many times you proofread.

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*Some people refer to them as relative clauses. Both names are correct and they mean the same thing.

**A common error that students make is putting the clause at the end of the sentence, like:

• Barack Obama is the 45th President of the United States, who grew up in Hawaii.

The adjective clause is describing Barack Obama, not the President, so it must move to the front of the sentence, as in:

• Barack Obama, who was born in Hawaii, is the 45th President of the United States.