Which and that are often used interchangeably. Choosing the wrong one is a common grammatical error.
Both are used to add extra information, called a clause, to a sentence. This type of clause modifies the noun that comes before it. It’s called a relative clause because it relates to the subject of your sentence. Traditionally, that is used when you add essential information, and which is used for nonessential extras.
In technical terms, these are called restrictive clauses (for essential information) and nonrestrictive clauses (for additional / nonessential information). Let's take a closer look at each type of clause.
A restrictive clause is also known as an essential clause or a defining clause. Without this clause, your sentence would be hard to understand.
- The windows that were washed last week are still clean.
- The dress that fit me so well when I bought it is now a little tight.
- That's the car that hit the cyclist.
The restrictive clause provides the essential extra detail we need to understand each sentence.
Traditionally, that is the only word used to start a restrictive clause. However, Merriam Webster points out that it increasingly acceptable to use either that or which to start a restrictive clause, especially in British English. As a rule of thumb, you will always be correct if you use that for a restrictive clause.
A nonrestrictive clause is also known as a nonessential clause or a non-defining clause. This clause adds additional, non-essential information to the sentence. If you remove it, the meaning is still clear.
- The windows have been cleaned, which makes the house look smart.
- The dress I bought is now a little tight, which is really annoying!
- This car, which is three times more expensive than the average vehicle, is the one that hit the cyclist.
Whilst the extra information adds interest, the core meaning of the sentence isn't lost if we remove the clause.
A nonrestrictive clause always starts with the word which. You can’t use that and which interchangeably for this type of sentence. It's also necessary to use a comma if the nonrestrictive clause is at the end of the sentence, or a pair of commas if it is in the middle of the sentence.
The Exception to the Rule
If you are referring to a person or an animal with a name, remember different rules apply. Don’t use that or which at all. Instead, use who or whom.
How to Use That in a Sentence
Simple sentences can be confusing. Adding extra details makes them clear to understand. This is especially true when you are referring to specific items within a group.
Use this simple sentence as an example:
- Houses are a good investment.
It’s clear and easy to understand the meaning. But are all houses a good investment? Adding in a clause would help clarify.
- Houses that are near public transport are a good investment.
This reason is essential to your argument.
How to Use Which in a Sentence
Let's look at another starter sentence:
- My house was a good investment because it has doubled in price since I bought it.
Here, the argument is already complete. Any additional information is non-essential.
- My house, which was built in 1950, was a good investment because it has doubled in price since I bought it.
The purpose of this sentence is that my house was a good investment. The year it was built is incidental. It's the same strength of argument either way; the nonrestrictive clause just adds extra information.
Why Does Getting It Right Matter?
Getting these clauses correct means your writing is clear to understand. For some professions such as the legal sector, making a mistake with a clause could have serious ramifications.
Understanding relative clauses is simple when you remember information added is essential or nonessential. If the information is necessary to the sentence structure, meaning, or argument, use that to start the clause. If it's an unrelated piece of additional information, use which.