People often ask whether they should use “which” or “that.” Some people even think these words are interchangeable, but they are not. In fact, the answer is very easy: You should use “which” to add non-essential information and “that” to add essential information.
Let’s explore the issue in more detail.
Sometimes in English we want to add more information to our language, so we use relative clauses. These function like adjectives to give more information about a noun.
Let’s take the noun “man” and use this example sentence:
Please go and find the man.
There is a problem here. Out of context, we do not know who this man is. He could be anyone!
When this happens, we should add more information by using a relative clause:
Please go and find the man that I told you about.
In this case, we have added some more information with “that” because “that” begins an essential relative clause. This extra information is necessary to fully understand the first part of the sentence, so it is essential.
When the information is not essential, we can use a non-essential relative clause instead. For example:
Please go and find my dog.
In this case, “my dog” refers quite specifically to one dog. Therefore, we do not need to add anything else for it to make sense. However, if we did add something else, we would use a non-essential clause. For example:
Please go and find my dog, which has been missing for the past hour.
You can see that I have added more information but it is not strictly necessary to the meaning of the first part.
When you add information to a noun, you need to think about whether the noun could have been understood without the extra part.
If you are wondering whether to use “which” or “that” then you need to think about the noun you are describing. Does it refer to one specific thing or is it one non-specific thing?
- my first car (one specific thing)
- the man (one non-specific thing)
- the second house on the left (one specific thing)
- the big house (one non-specific thing)
Let’s take the two specific things and make some practice sentences:
- I am often reminded of my first car.
- I used to live in the second house on the left.
These sentences are completely self-contained. We do not need to add anything else. However, we can add extra information by using the word “which” and also a comma. For example:
- I am often reminded of my first car, which was green.
- I used to live in the second house on the left, which used to have a big tree in the front yard.
Notice that each relative clause is separated from the first part with a comma and then the word “which” is used. This is how we deal with non-essential relative clauses.
Essential Relative Clauses
Let’s look at those two non-specific things: a man and a big house. We can make some more practice sentences:
- Did you see the man?
- Just walk past the big house.
Here, there is a slight problem. Out of context, these sentences have little meaning. If you said them, someone might reply, “Who is the man?” or “What big house?”
This means that any further information you give to explain the man or the house would be considered essential. For example:
- Did you see the man that waved at us?
- Just walk past the big house that’s on the corner of Main Street.
Which or That?
When you are writing and ask yourself, “Should I use ‘which’ or ‘that’?” you should stop for a moment and ask yourself, “Is this extra information necessary to understand the noun?” If yes, you must use “that.” If no, you must use “which.”
Which, That, and Commas
One final word on this subject: Please use commas correctly.
When you use “which” it must follow a comma.
When you use “that” there must be no comma.