Defining clauses specify which person or thing is being referred to, while non-defining clauses add extra information about someone or something already identified. Both are types of relative clauses that modify nouns, but they differ in punctuation, placement, and necessity.
Defining clauses are essential to the meaning of a sentence. Remove one, and the sentence loses clarity or changes meaning. Non-defining clauses provide supplementary detail and can be removed without altering the core meaning. The difference lies in whether the clause is necessary to identify the noun it modifies.
For example: "The book that I borrowed from the library is overdue." Here, "that I borrowed from the library" defines which book is overdue. Without it, the sentence doesn't specify which book. Contrast this with: "My brother, who lives in Seattle, is visiting next week." The clause "who lives in Seattle" adds context but isn't needed to identify the brother-he's already known as "my brother."
Defining clauses follow the noun they modify and are introduced by relative pronouns such as that, which, who, whom, whose, or when. They are not set off by commas because they are integral to the sentence's meaning.
Non-defining clauses are set off by commas and usually begin with which, who, or whose. They offer additional details-like location, appearance, or status-but do not narrow down the identity of the noun. Their presence is optional; removing them leaves the sentence grammatically complete.
Consider: "The woman who spoke at the conference won an award." The clause "who spoke at the conference" identifies the specific woman among others. Now remove the clause: "The woman won an award." The sentence still makes sense and retains its core meaning. But in "My neighbor, who has three dogs, is very friendly," removing the clause gives "My neighbor is very friendly"-still correct, but less informative.
Use commas to separate non-defining clauses from the rest of the sentence. Omit them in defining clauses. This rule applies regardless of the relative pronoun used.
| Type | Punctuation | Relative Pronoun | Example |
|---|---|---|---|
| Defining | No commas | that, which (for things), who (for people) | The car that broke down was old. |
| Non-defining | Commas before and after | which, who, whose | My sister, who lives in Boston, is a teacher. |
A common mistake is using commas with defining clauses. For example: "The student who failed the test, was surprised." This incorrect punctuation suggests the clause is non-defining, implying the student was already known. But the intended meaning is that the student who failed is the one who was surprised. Correct version: "The student who failed the test was surprised."
Another error: replacing "that" with "which" in defining clauses. While "which" can sometimes substitute for "that," it's not standard in formal writing. Use "that" for defining clauses, "which" for non-defining ones. The sentence "The house which we bought last year is expensive" sounds awkward. Better: "The house that we bought last year is expensive."
Many learners confuse non-defining clauses with appositives. An appositive renames a noun directly. For example: "My friend Sarah is coming to dinner." "Sarah" is an appositive. A non-defining clause adds descriptive detail: "My friend, who is a chef, is coming to dinner." The clause is not renaming the noun-it's describing it.
Also, non-defining clauses cannot use "that." You cannot say: "My father, that works at the hospital, is tired." That construction is ungrammatical. Only "who," "which," or "whose" may introduce non-defining clauses.
One non-obvious distinction: non-defining clauses often use "who" for people, even when the person is not the subject. In "My teacher, who gave me a B+, is retiring," "who" refers to "teacher," not "me." The pronoun agrees with the antecedent, not the speaker.
Defining clauses appear frequently in news headlines and technical writing. For example: "The suspect who entered the bank was arrested." The clause specifies which suspect-only the one who entered the bank. Removing it creates ambiguity: "The suspect was arrested" could refer to any suspect.
Non-defining clauses are common in narrative writing and formal reports. Consider: "The Eiffel Tower, which stands 300 meters tall, is a symbol of Paris." The height is extra context. The sentence remains valid without it: "The Eiffel Tower is a symbol of Paris."
Here's a fully worked example:
"The dog that bit the mail carrier was taken to the vet."
Analysis: "That bit the mail carrier" is a defining clause. It identifies the specific dog among many. Without it, "The dog was taken to the vet" lacks specificity. The sentence relies on the clause to convey meaning.
"My dog, which has a white patch on its chest, loves walks."
Analysis: "Which has a white patch on its chest" is non-defining. The dog is already identified as "my dog." The clause adds visual detail. Removing it: "My dog loves walks" still works and keeps the core idea.
As a writer, I once misused a defining clause in a job application letter. I wrote: "I have experience in project management, which includes budget oversight." I meant to describe my skills. But "which" implied the entire experience was limited to budget oversight. I corrected it to: "I have experience in project management, including budget oversight." The revised version uses a non-defining clause correctly, avoiding confusion.
Use defining clauses when you need to distinguish between multiple possibilities. Use non-defining clauses when adding color, context, or background.
For academic writing, define your terms. If you're discussing "the policy that affects rural schools," you're referring to a specific policy. If you write "the policy, which affects rural schools, is under review," you're stating a fact about a policy already mentioned.
When editing, ask: "Would the sentence lose meaning if I removed this clause?" If yes, it's defining. If no, it's non-defining. This simple test prevents most errors.
Understanding the difference improves clarity in both spoken and written English. Misplaced commas or incorrect pronouns can change the meaning entirely. Always verify the clause's role in the sentence.