Differences Defining and Non-Defining Relative Clauses

The difference between defining and non-defining relative clauses is important to understand if you want to use relative clauses correctly.

Relative clauses add extra information to a sentence by defining a noun. However...

Defining relative clauses add essential information to a sentence and the clause cannot be omitted.
Non-defining relative clauses add non-essential information to a sentence and can be left out of a sentence without changing its meaning.

Let's take a closer look with some examples.

What is the difference between a defining and non-defining relative clause?

Defining relative clauses are crucial to the sentence. If the clause is removed, the sentence will not make sense and will not have the same meaning.

That is why they are also called essential relative clauses, essential meaning absolutely necessary or extremely important.

Let's take a look with some examples! (The relative clause is in red).

Defining Relative Clauses:

That is the woman who stole my handbag.
This is the book that I read.
The car that I drive is a red Mercedes.

Now lets see what happens if we take out the relative clause:

That is the woman.
This is the book.
The car is a red Mercedes.

As you can see the, the meaning of the sentences have been completely changed, or they don't really mean anything.

We now have questions about the noun:

The woman who what?
The book that what?
What car?

Non-defining relative clauses, however, are not essential to the sentence. If the clause is removed, the sentence still makes sense and has the same meaning.

The information may of course be relevant, which is why it is included, but it is not the main point that the sentence is trying to convey about the noun.

Non-Defining Relative Clauses:

My grandmother, who is 86 years old, is coming on holiday with us this year.
My car, which a Mercedes, can reach speeds of 140 km per hour.
Canberra, which is the capital of Australia, is located at the northern end of the Australian Capital Territory

If we now take out the non-defining relative clauses, the main idea of the sentence remains and the sentence still makes perfect sense.

My grandmother is coming on holiday with us this year.
My car can reach speeds of 140 km per hour.
Canberra is located at the northern end of the Australian Capital Territory.

What about commas in relative clauses?

You may have noticed another difference between defining and non-defining relative clauses, and that is with commas.

We'll look at commas and relative clauses in more detail in another lesson, but basically defining relative clauses have no commas and non-defining do have commas:

Defining relative clause:

My plan is to buy the house that costs the least money.

Non-defining relative clause:

I'll take the bus, which I think arrives in about ten minutes.

What about 'that' in relative clauses?

'That' is another important distinction in the difference between defining and non-defining relative clauses.

You may have noticed that some relative clauses use 'that' while others do not. The basic difference is that:

In defining relative clauses the words who or which can be replaced by that
In non-defining relative clauses that cannot be used.

In fact, it is much more common to use that instead of which in defining relative clauses, but it can also replace who.

'That' in Defining Relative Clauses:

It is my doctor who/that advised me to lose weight.
I'll take the hat which/that fits me best.
My boss is the person who/that usually leaves the office last

More on Relative Clauses:

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