Which vs. That: Use This Test to Know

Have you wondered which of the three sentences is correct?

We went to the restaurant which Lucy recommended.

We went to the restaurant, which Lucy recommended.

We went to the restaurant that Lucy recommended.

Third is unambiguously correct, but first too is accepted. In this post, we’ll learn the rules and real-world practice behind.

The relative pronouns which and that are both used for non-humans and are sometimes used interchangeably. Hence, they confuse many as to which to use when. Most go by what sounds right in a sentence, but that’s not full proof. Let’s dig out details.

Who and whom can create similar confusion. Learn more:

Whereas that is used to introduce restrictive relative clauses, which is used to introduce non-restrictive relative clauses. (Restrictive clauses are also called defining or essential clauses, and non-restrictive clauses are also called non-defining or non-essential clauses.) Examples:

(Note: Comments that go with examples are in square brackets. Second, relative clauses have been underlined at some places for clarity.)

Tom’s car that he bought last year met with accident today. [Comment: Restrictive clause]

Tom’s car, which he bought last year, met with accident today. [Non-restrictive clause]

In the first sentence, we’re describing a situation where Tom owns several cars. So, we need to identify the particular car that met with accident. A restrictive clause does the job of narrowing down choices to arrive at a particular person or thing. Here, the clause that Tom bought last year does that job. Without the restrictive clause, we can’t identify the car among many:

Tom’s car met with accident today. [Now, it could be any of Tom’s several cars. The meaning we want to convey is clearly lost.]

In the second sentence, we’re describing a situation where Tom owns just one car. Because Tom owns just one car, we don’t need to identify it. Hence, we don’t need the information on when the car was bought. But why not use that real estate to squeeze in some extra information about the car? It’s not required, but it can provide the readers more information about the car. Herein enters non-restrictive clause. Where we don’t need to identify (or narrow down), we use a non-restrictive clause to add extra information. Here, the relative clause which I bought last year does that job. Note that it comes with a pair of commas. Without the non-restrictive clause, we can identify the car because there is just one:

Tom’s car met with accident today. [The meaning we want to convey remains unchanged because Tom has just one car.]

There is no overlap in the use of that and which: that is used to start restrictive clauses and which is used to start non-restrictive clauses.

Learn more on restrictive vs. non-restrictive:

Test: How to decide between which and that?

Drop the relative clause. If the specificity (or identity) of the noun changes, you’ve to use that because the clause is restrictive. If the specificity of the noun doesn’t change, you’ve to use which (along with commas) because the clause is non-restrictive.

Let’s apply this test to the example we considered earlier.

Tom’s car which/that he bought last year met with accident today.

The test will determine which of the two to use here.

Consider the same two scenarios we considered earlier. (Note that when you’re writing on any topic, you’ll always know the background information such as number of cars Tom owns.)

Scenario 1: Tom has five cars

On dropping the relative clause, we get:

Tom’s car met with accident today.

Now, we don’t have the identifying information. Someone reading this sentence wouldn’t know which of the five cars met with accident. Without the relative clause, specificity is clearly lost. Hence, the sentence will require that.

Tom’s car that he bought last year met with accident today.

Scenario 2: Tom has one car

On dropping the relative clause, we get:

Tom’s car met with accident today.

Someone reading this sentence doesn’t need to be informed about which car met with accident because there is just one car. Without the relative clause, specificity isn’t lost. Hence, the sentence will require which.

Tom’s car, which he bought last year, met with accident today.

Examples of deciding which vs. that

In the examples below, we start undecided on which vs. that and without commas. We then freeze one and put commas if the answer is which. Some of the sentences are also provided with background information in square brackets next to them. You can also apply the test we covered to arrive at the same answer.

1. The city’s airport which/that was recently renovated is seeing an all-time high traffic. [The city has one airport.]

Since the city has one airport, we don’t need to identify it. Hence, we use non-restrictive clause starting with which. And don’t forget the comma!

The city’s airport, which was recently renovated, is seeing an all-time high traffic.

2. The city’s airport which/that was recently renovated is seeing an all-time high traffic. [The city has two airports.]

Since the city has two airports, we need to identify which airport has seen a surge in traffic. Hence, we use restrictive clause starting with that. And you don’t need a comma here because the clause is restrictive.

The city’s airport that was recently renovated is seeing an all-time high traffic.

3. Diamond which/that is pure carbon is produced by intense heat and under great pressure.

All diamonds are produced by heat and pressure. Hence, we don’t need to identify diamond. If there was a diamond that was produced by, say, chemical reaction, then we need a restrictive clause starting with that to identify the one that is produced by heat and pressure. But not in this case.

Diamond, which is pure carbon, is produced by intense heat and under great pressure.

4. The pictures which/that are being sent back by Perseverance Rover may change our understanding of Mars.

Pictures can be from anywhere: a rover on a planet other than Mars or some photographer. But not all pictures are changing our understanding of Mars. Therefore, we need a restrictive clause starting with that to identify the pictures.

The pictures that are being sent back by Perseverance Rover may change our understanding of Mars.

5. We went to Chef’s Table which/that Lucy recommended.

The moment we name a restaurant, it’s identified, and we don’t need to further narrow it down. Therefore, we use a non-restrictive clause starting with which.

We went to Chef’s Table, which Lucy recommended. [There is only one comma because the clause comes at the end of the sentence.]

6. We went to the restaurant which/that Lucy recommended.

Here we need to identify the restaurant by restricting it to the one recommended by Lucy. That’s why we’ve used a restrictive clause.

We went to the restaurant that Lucy recommended.

7. Statue of Liberty which/that was gifted from the people of France is a colossal sculpture located on Liberty Island in New York City.

There is only one Statue of Liberty, and hence we don’t need to further narrow it down through a restrictive clause. That’s why we use non-restrictive clause to provide extra information.

Statue of Liberty, which was gifted from the people of France, is a colossal sculpture located on Liberty Island in New York City.

You can practice more such sentences in the exercise at the end.

What happens in practice though?

The stipulation that that can be used only for restrictive clauses and which can be used only for non-restrictive clauses is more strictly followed in American English than in others. American style guides such as The Associated Press Stylebook and The Chicago Manual of Style prescribe avoiding which for restrictive clauses. (Note that this isn’t a grammar rule; it’s only a style recommendation.)

So, you may be corrected if you write these sentences in U.S.:

We went to the restaurant which Lucy recommended.

The pictures which are being sent back by Perseverance Rover may change our understanding of Mars.

Because of recommendation of style guides and other reasons, that is winning the popularity war. As per the study by Lars Hinrichs et at. 2015, since 1990s, that has been gradually replacing which in restrictive relative clauses, with American English spearheading this change.

However, in practice, especially outside American English, which can also be used for restrictive clauses, like in the above two examples.

How understanding which vs. that helps your writing?

If you understand the difference between relative pronouns that and which, you won’t write a sentence like:

We went to the restaurant, which Lucy recommended.

You’ll also understand why someone changed We went to the restaurant which Lucy recommended to:

We went to the restaurant that Lucy recommended.

Last, nailing errors in which (whether restrictive or not) in a piece of writing isn’t straightforward like fixing spelling or punctuation. You need to go on a which-hunt (ctrl + F) and give a hard look at each which. Reliance on how it sounds in a sentence may lead you astray.

Exercise on which vs. that

For each sentence below, pick which or that along with correct punctuation. You can stick to use of that for restrictive clause and which for non-restrictive clause.

1. Too much money can lead to vices and bad habits which/that ruin people.

2. As a result, wall-mounted device which/that people can install at home was born.

3. I would advise you to work in a market which/that is growing fast.

4. Russia has the highest number of Siberian tigers in the wild which/that are the largest members of tiger family.

5. The historian has provided an interpretation of the inscriptions which/that is now generally accepted.

Answers to Exercise

1. Too much money can lead to vices and bad habits, which ruin people.

Vices and bad habits don’t come in different shapes and sizes, so we don’t need to narrow them down further, implying non-restrictive clause (and hence which and comma).

2. As a result, wall-mounted device, which people can install at home, was born.

The context doesn’t provide information about multiple types of wall-mounted devices, and hence we don’t need to distinguish one type from the other. Therefore, it’s a non-restrictive clause. If there were two types of wall-mounted devices – one that can be installed by people and the other that requires a technician – then our sentence would be: As a result, wall-mounted device that people can install at home was born.

3. I would advise you to work in a market that is growing fast.

There are multiple markets, and we need a restrictive clause to identify one.

4. Russia has the highest number of Siberian tigers in the wild, which are the largest members of tiger family.

We’ve already identified Siberian tigers among other species of tigers, so we don’t need to further narrow it down through a restrictive clause.

5. The historian has provided an interpretation of the inscriptions, which is now generally accepted.

The historian has provided an interpretation, which doesn’t need further identification, so the information about its acceptance is just extra information expressed through a non-restrictive clause.

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