Defining and non-defining relative clauses connect grammar, punctuation, and meaning. A defining clause helps identify the person or thing being discussed; a non-defining clause adds information about an already identified referent. This article compares the two patterns, relative words such as who, which, and that, and preposition placement. Compare “The employees who passed the audit received bonuses” with “The employees, who passed the audit, received bonuses.” The first identifies employees by their audit result. The second presents passing the audit as additional information about the employees already under discussion. The relevant group comes from context; neither sentence necessarily refers to every employee in an entire company.
What defining and non-defining relative clauses do
A defining relative clause, also called a restrictive relative clause, is essential to the meaning of the sentence. It narrows the noun it follows and tells the reader exactly which person, thing, or group is meant. In “The book that explains phrasal verbs is on my desk,” the clause “that explains phrasal verbs” identifies the book. Remove it, and the listener no longer knows which book. A non-defining relative clause, also called a nonrestrictive relative clause, adds extra information about a noun that is already identified. In “My desk, which faces the window, is covered with notes,” the clause “which faces the window” gives extra detail about the desk. Remove it, and the core reference still stands: my desk is covered with notes.
Defining relative clauses are not separated from their head noun by commas. Non-defining clauses are marked off with a comma at the start and, if the main sentence continues, a closing comma. These marks distinguish integrated identification from supplementary information. Compare “Students who revise consistently score better” with “Students, who revise consistently, score better.” The first limits the statement to students who revise consistently; it does not independently establish what happens to other students. The second presents consistent revision as a generalization about the students being discussed. These are grammar examples, not evidence for an actual learning outcome.
Which, that, who, whom, and whose: choosing the right relative word
For things, that and which can introduce defining relative clauses. For non-defining clauses referring to things, use which, not that. For people, who works in both types, while that is also possible in defining clauses. Formal object uses may take whom; whose expresses a possessive relationship. Compare “The software that reduces image size is expensive” with “The software, which reduces image size automatically, is expensive.” In the first, the function identifies the software; in the second, it adds information. Likewise, “The analyst who prepared the report will present first” identifies an analyst, while “Marta, who prepared the report, will present first” adds information about Marta.
Some house styles prefer that for defining clauses referring to things and reserve which for non-defining clauses. Following that convention can make a document consistent, but it should not be presented as a universal grammar rule. Defining which is standard, especially in British usage. First decide whether the clause identifies the referent or adds a supplement, then choose a grammatical relative word and follow any required house style.
Mandatory commas and how punctuation changes meaning
To choose the punctuation, ask what the clause does in this context. In “Cars that fail inspection cannot be registered,” it identifies the cars covered by the statement, so there are no commas. In “My car, which failed inspection, cannot be registered,” assume the reader already knows which car is meant: the inspection detail is supplementary. A non-defining clause in the middle needs opening and closing commas; at the end, it needs its opening comma and the sentence-ending punctuation. A possessive such as my does not by itself make every following relative clause non-defining.
Relative-clause punctuation can change the group a statement concerns. “The managers who worked remotely attended by video” makes a statement about the remotely working managers; it does not tell us how other managers attended. “The managers, who worked remotely, attended by video” adds that the managers already being discussed worked remotely. The second sentence concerns all managers in that context, not automatically every manager in the organization. Match the punctuation to the intended reference rather than assuming a wider claim.
How relative clauses connect to clauses and prepositions
This hub also sits inside the broader study of clauses and prepositions, because relative clauses regularly interact with prepositional structures. Many relative clauses contain a stranded preposition, especially in speech and general prose: “The colleague who I worked with now lives abroad.” In more formal style, the preposition may move before the relative pronoun: “The colleague with whom I worked now lives abroad.” Both are grammatical. The difference is register. Stranding is common, natural, and fully accepted in modern English. Fronting the preposition sounds more formal and can become heavy if overused. The same choice appears with things: “The policy that we argued about” versus “the policy about which we argued.”
Understanding this connection helps learners unify several grammar topics instead of memorizing isolated rules. A relative clause is still a clause, so it has grammatical roles inside it: subject, object, complement, or object of a preposition. In “The researcher who led the trial published the results,” who is the subject of led. In “The researcher whom the journal interviewed published the results,” whom is the object of interviewed. In “The researcher to whom the grant was awarded published the results,” whom is the object of the preposition to. Once students see function, punctuation choices become easier because sentence structure becomes visible.
Common error patterns and practical fixes
Check the following patterns when editing. Some are grammatical errors, while others depend on the intended meaning and context:
| Error pattern | Example to check | Revision for the stated meaning | Why it matters |
|---|---|---|---|
| Non-defining clause without commas | My laptop which I bought in 2022 still runs well. | My laptop, which I bought in 2022, still runs well. | If the laptop is already identified, the purchase detail is extra. If it selects one of several laptops, a defining clause without commas can be appropriate. |
| Defining clause with commas | Books, that cover verb patterns, help advanced learners. | Books that cover verb patterns help advanced learners. | The clause identifies a subset of books. |
| That in a non-defining clause | The policy, that was revised last week, now applies. | The policy, which was revised last week, now applies. | Standard edited English uses which, not that, in non-defining clauses. |
| Wrong scope caused by commas | Editors, who check facts carefully, catch these errors. | Editors who check facts carefully catch these errors. | The version with commas generalizes about the editors under discussion. The revision limits the statement to editors identified by careful fact-checking; choose the intended meaning. |
In “The book I recommended is out of print,” the relative word can be omitted because it would be the object of recommended. “My dictionary I recommended to the class is out of print” can also have a defining reading: among my dictionaries, this is the one I recommended. If one dictionary is already identified and the recommendation is extra information, write “My dictionary, which I recommended to the class, is out of print.” Keep the relative word in this non-defining version. Possessives do not settle the choice; the intended reference does.
Style, usage, and guidance for grammar learning
For a grammar hub, the practical takeaway is to teach relative clauses together with noun identification, punctuation, and preposition placement. Start with meaning before terminology. Ask, “Do we need this clause to know which noun is meant?” If yes, it is defining. If no, it is non-defining. Then apply the pronoun choice. Use that or sometimes which for defining things depending on house style, and use which for non-defining clauses about things. Use who for people, whose for possession, and whom when formal object position calls for it. Finally, decide whether the preposition should stay at the end, which is normal in most contexts, or move before the pronoun for a more formal tone.
To practise, take a relative-clause sentence and identify its head noun, the relative word’s role, and the group being discussed. Decide whether the clause selects the referent or adds a supplement, then check its punctuation. Finally, compare any revised sentence with the original intended meaning. For further examples, see the British Council explanation of defining relatives and Cambridge’s relative-pronoun reference.
Frequently Asked Questions
What is the difference between a defining relative clause and a non-defining relative clause?
A defining relative clause limits or identifies the person, thing, or group meant. “The students who studied passed the exam” says that the students identified by studying passed. It does not tell us whether any other students passed. No commas separate this identifying clause from students. A non-defining clause instead adds a supplement about a referent identified in context.
A non-defining relative clause, by contrast, adds extra information about a noun that is already clear and identifiable. If you remove the clause, the core meaning of the sentence remains intact. For example, “My car, which I bought last year, is already having problems” refers to one specific car that is already known to the reader or listener. The clause “which I bought last year” simply adds detail, so it is enclosed in commas.
This distinction matters because it affects grammar, punctuation, and meaning all at once. The writer is making a decision about whether the clause limits the noun or merely comments on it. That is why relative clauses are such a common trouble spot: the issue is not just word choice, but whether the information is essential or parenthetical.
When should I use “that” and when should I use “which” in relative clauses?
In standard edited English, especially in American usage, “that” is commonly used to introduce defining relative clauses, while “which” is typically used to introduce non-defining relative clauses. So you would usually write “The book that explains the rule is on the desk” when the clause identifies a particular book, but “This book, which explains the rule clearly, is on the desk” when the book is already identified and the clause simply adds extra information.
This convention is helpful because it makes meaning clearer at a glance. “That” tends to signal restriction or identification. “Which,” when paired with commas, tends to signal additional commentary. Many style guides encourage this distinction because it reduces ambiguity and makes prose easier to parse.
Defining which is grammatical, particularly common in British English, and is not automatically a missing-comma error. A house style may prefer defining that; follow it when required. For a non-defining clause about a thing, use which with the appropriate commas. The deciding question is the clause’s function, not a rule that every which must follow a comma.
Why are commas mandatory with non-defining relative clauses?
Commas are mandatory with non-defining relative clauses because they signal that the clause is not essential to identifying the noun. In other words, the commas mark the clause as supplementary information. They tell the reader, “This is extra detail, not part of the core identification.” Without those commas, the sentence can suggest a defining meaning instead, which may alter what the writer appears to mean.
Consider the difference between “My brothers, who live in Chicago, are visiting” and “My brothers who live in Chicago are visiting.” In the first sentence, the commas suggest that all of my brothers live in Chicago, and the clause merely adds information about them. In the second, the lack of commas suggests that only the brothers who live in Chicago are visiting, perhaps in contrast to brothers who live elsewhere. The punctuation changes the logic of the sentence.
This is why the commas are not decorative. They are part of the grammar. A non-defining clause is parenthetical by nature, and the punctuation must reflect that parenthetical status. If you omit the commas, you risk implying a narrower, defining meaning than you intended. In formal writing, this is one of the clearest examples of punctuation directly affecting interpretation.
Can removing or adding commas really change the meaning of a sentence?
Yes, absolutely. In sentences with relative clauses, commas often determine whether the clause is restrictive or non-restrictive, and that distinction can change the meaning in a significant way. This is not a minor stylistic preference. It is a structural choice that tells the reader whether the information identifies the noun or merely comments on it.
“Employees who arrive late will be warned” applies the warning to employees identified by arriving late. It does not say what other conduct might also lead to a warning. “The employees, who arrived late, were warned” presents lateness as additional information about the employees already identified in context. Those employees were warned; the sentence does not automatically make a claim about the organization’s entire workforce.
For this reason, writers and editors should never treat these commas casually. If the clause answers the question “Which one?” or “Which ones?” it is likely defining and should not be enclosed in commas. If the clause merely adds a side note about something already identified, it is non-defining and should be set off with commas. A quick test is to remove the clause: if the sentence still clearly identifies the noun, commas are probably appropriate; if the identity becomes unclear or broader than intended, the clause is probably defining.
What are the most common mistakes people make with defining and non-defining relative clauses?
One pattern to check is choosing commas and a relative word before deciding what the clause means. Use the intended reference first: does the clause identify the person or thing, or add information about it? Then choose a suitable relative word. Do not assume that which always requires commas or that that can introduce a supplementary clause.
Another frequent mistake is mixing the signals. For example, writers sometimes use “that” with commas, or use “which” without commas when they intend a non-defining meaning. In careful standard usage, those combinations often create awkwardness or ambiguity. A clean pattern helps: defining clause, no commas, often “that”; non-defining clause, commas, usually “which” or “who.”
A third problem is overpunctuation or underpunctuation in long sentences. When a writer is juggling several ideas, the relative clause may be punctuated according to rhythm rather than grammar. That can make a sentence sound fine when spoken but read inaccurately on the page. The best remedy is to identify the noun first, then ask whether the clause is narrowing it down or simply describing it. Once that is clear, the correct punctuation usually follows naturally.
An editing check is to read the sentence in its actual context, identify the noun phrase, and compare the intended referent before and after the relative clause is removed. Removal can change identification or discard supplementary information. Neither grammaticality after removal nor a possessive determiner alone settles the punctuation. Explain the meaning you intend before changing the commas.
