Defining clauses define the noun
Defining clauses introduce additional information to a sentence by defining the noun. That is why they are called defining clauses.
Take a look at this sentence: This is the house that Jack built.
The defining clause is that Jack built. It defines the house.
Defining clauses belong to the category subordinate relative clauses.
There are two kinds of subordinate relative clauses:
• Defining clauses. These are usually introduced by that, but also by which or who.
• Non-defining clauses. These are descriptive and provide information about someone or something, but this information doesn’t exactly define what is being written about. Without them the sentence still makes sense and is grammatically correct. They are introduced by relative pronouns (who, whom, whose, which) which refer to a noun that appears earlier in the sentence e.g. My brother, who is older than me, owns a sports car. Note that they are never introduced by the word that. Commas (or parentheses) are used to distinguish non-defining clauses from the rest of the sentence.
Difference between defining clauses and non-defining clauses
These sentences highlight the difference between defining clauses and non-defining clauses:
(a) I dislike travelling in buses that are dirty.
(b) I dislike travelling in buses, which are dirty.
Sentence (a) means that I dislike travelling in dirty buses, and by implication that I am happy to travel in clean buses. This is an example of a sentence with a defining clause. If the defining clause is omitted, the basic meaning of the sentence would be changed from I dislike travelling in some buses to I dislike travelling in all buses.
Sentence (b) means all buses are dirty and by implication that I would rather use other means of transport.
Sentence (a) is an example of a sentence with a defining clause, and sentence (b) is an example of a describing or non-defining clause. If the describing clause is removed from this sentence the basic sense of it remains the same i.e. I dislike travelling in all buses.
A non-defining or describing clause is normally introduced by a comma. A defining clause is essential to the sentence and cannot be put within commas. It is important to note that it can’t be omitted without eliminating or severely limiting the sense of the sentence.
Defining clauses are usually introduced by that
Defining clauses are usually introduced by that e.g. I live in the house that Jack built.
Here the defining clause is so fundamental that if it were to be omitted, the sentence would be incomplete and confusing. It defines the house as being the only house built by Jack.
In the sentence I live in a house, which Jack built the meaning is that Jack has possibly built several houses and I live in one of them. This is an example of a non-defining (or describing) subordinate clause.
Points to remember
• Who and which may also be used to introduce defining clauses but that is almost always preferred.
• That is used to introduce defining clauses only.
• That always defines rather than merely describes.
• Defining clauses must not be marked off with commas.
• Who, and sometimes that, refers to people, but that and which refer to things.
Defining clauses are usually introduced by the word that, and, unlike non-defining clauses, are essential to the meaning of the sentence.
Tana French (1973)
Tana French is an Irish novelist and theatrical actress.
“Don’t get discouraged if you’re hammering away at a sentence or a paragraph or a chapter, and it keeps coming out wrong. You’re allowed to get it wrong, as many times as you need to; you only need to get it right once.”