Relative Clauses at C2

Learning outcomes

You learned relative clauses years ago. You join two sentences with “who” or “which” without thinking, you know “that” can stand in for either, and you can write a sentence three clauses deep. And yet a reviewer once moved a single comma in your design doc and quietly changed who you were talking about, and you could not have said why. The shapes are easy. The fine choices, where one comma flips the meaning and one wrong word makes a person sound like a thing, are what separate correct English from precise English. This page is about those choices.

After studying this page, you can:

• Tell a defining clause from a non-defining one by what each does to meaning, and place or withhold the comma so the sentence says what you intend.
• Choose between “that” and “which” for defining clauses, and explain why an American editor and a British editor would mark the same sentence differently.
• Drop the relative pronoun when it is the object, and reduce a full relative clause to a tight participle phrase, without losing meaning.
• Place a preposition by stranding it at the end or pied-piping it to the front, and pick the register each one signals.
• Use sentential “which” to comment on a whole clause, and handle “whose,” “where,” “when,” and “why” as relatives.
• Spot and fix the comma errors, the resumptive pronouns, and the person-as-thing slips that survive into advanced writing.

Before we dive in

Here is the sentence that started a small argument on a code review. An engineer wrote, in a postmortem: “The engineers who were on call missed the alert.” A reviewer changed it to “The engineers, who were on call, missed the alert,” and added a note: “just adding the commas back.” But the two sentences do not mean the same thing, and the reviewer had silently rewritten the finding.

The first sentence, with no commas, says that among all the engineers, the particular subset who happened to be on call missed the alert. It singles them out. It implies other engineers existed who were not on call and who are not being blamed. The second sentence, with commas, says that the engineers, all of them, missed the alert, and adds, in passing, that they all happened to be on call. The clause is now a remark on the side, not a filter. One reading points a finger at a few people; the other points it at everyone and mentions on-call status as a footnote. A pair of commas did that.

This is the trap of relative clauses at an advanced level. The grammar of joining clauses is something you mastered long ago. What stays hard is that a relative clause does one of two completely different jobs, that the punctuation is the only outward sign of which job it is doing, and that several further choices, “that” or “which,” keep the pronoun or drop it, strand the preposition or move it, carry real meaning and real register. A grammar checker will pass both versions of the on-call sentence, because both are grammatical. Only a reader who knows the machinery will notice that they accuse different people. That machinery is what we build here, starting with the two jobs and the comma that tells them apart.

The two relative clauses and the comma that splits them

Every relative clause is doing one of two jobs, and English marks the difference with commas alone. The first job is to define: to narrow down which thing or person you mean, out of several it could be. The second job is to add a remark: to take a thing already fully identified and tell you something extra about it, in passing. The first is called a defining (or restrictive) clause; the second is a non-defining (or restrictive’s opposite, non-restrictive) clause. Get the comma wrong and you switch jobs, and the sentence changes who or what it is about.

Start with the defining clause, the one with no commas. “The file that I deleted was the config.” Here “that I deleted” is doing essential work: it tells you which file, out of all the files, you mean. Remove it and the sentence collapses into “the file was the config,” which file? The clause is welded to the noun; it is part of identifying the thing. Because it is essential, no comma separates it, the way you would not put a comma in the middle of a name.

Now the non-defining clause, the one wrapped in commas. “The config file, which I deleted by mistake, broke the build.” Here the file is already identified (it is the config file), so “which I deleted by mistake” is not narrowing anything down. It is a parenthetical aside, an extra fact dropped in. You could lift the whole clause out, “the config file broke the build,” and lose only a detail, not the identity of the subject. The commas mark it as removable, the way parentheses would.

Two structural rules fall straight out of this difference, and both are absolute. First, a non-defining clause cannot use “that.” You can write “the file that I deleted” (defining) but never “the config file, that I deleted by mistake” (non-defining); the aside takes “which” or “who,” never “that.” Second, a non-defining clause cannot drop its pronoun. A defining clause often can (“the file I deleted”), but you can never reduce “the config file, which I deleted” to “the config file, I deleted.” The aside needs its “which” or “who” to stay anchored. We will see why the pronoun can vanish in the defining case shortly; for now, hold the contrast.

The table below lines the two clause types up feature by feature, so you can see that the comma is not a stylistic flourish but the load-bearing signal.

Read across the “Commas” and “Can it use ‘that’?” rows together and you have the core of the whole topic: the comma decides the job, and the job decides which words are even allowed. Everything else on this page is a refinement of these two clause types. The next sections take the choices one at a time, starting with the one that divides editors on two continents.

That versus which, and where the Atlantic divides them

Inside a defining clause, when the thing is not a person, you can usually choose between “that” and “which”: “the service that handles auth” or “the service which handles auth.” Both are grammatical, both are defining, and the meaning is the same. So the choice is one of style and regional preference, and this is the one corner of relative clauses where careful Americans and careful Britons genuinely disagree.

The American convention, championed by usage guides on that side, is a clean division of labor: use “that” for defining clauses and reserve “which” for non-defining ones. Under this rule, “the service that handles auth” (defining, narrowing) is correct, and “the auth service, which we wrote last year” (non-defining, aside) is correct, but “the service which handles auth” is to be edited to “that.” The comma and the word are made to agree: no comma goes with “that,” a comma goes with “which.” Garner’s usage guidance treats this that-for-restrictive rule as the house style of careful American editing, a convention worth following for the clarity it buys, even while acknowledging it is a preference rather than a law of grammar.

The British convention, descending from Fowler, is more relaxed. Fowler observed the that-which distinction and thought it would be tidy if everyone kept to it, but he conceded that good writers do not, and that “which” in a defining clause has a long and respectable history. So British editing tolerates “the service which handles auth” without flinching. The one rule both sides enforce absolutely is the non-defining one: nobody, on either continent, allows “that” in a clause set off by commas. The disagreement is only about whether defining “which” is a fault or a free choice.

To make the whole decision mechanical, the flowchart below routes any noun-plus-clause you are building to the right punctuation and the right relativiser. Walk a phrase down it and you will land on a single correct form.

The tree makes the dependencies visible: the comma question comes first, because it decides the job, and only after the job is fixed does the person-or-thing question and the style question come into play. Notice that two branches forbid “that” outright, both of them on the non-defining side. That is the rule with no exceptions; the that-versus-which choice only ever exists inside a defining clause about a thing.

Dropping the pronoun: the zero relative

English lets you delete the relative pronoun entirely in one specific situation, and the result sounds more natural than keeping it. “The file that I deleted” becomes “the file I deleted.” “The bug which we fixed” becomes “the bug we fixed.” “The person who you met” becomes “the person you met.” The pronoun simply vanishes, and the clause carries on without it. This is called the zero relative, or the omitted (contact) relative, and using it well is one of the marks of a fluent writer, because the full pronoun, while never wrong, often reads as slightly heavy.

The rule for when you can drop it is precise: you can omit the relative pronoun only when it is the object of its clause, not the subject. Test it by finding the verb inside the relative clause and asking what the missing pronoun was doing. In “the file [that] I deleted,” the subject of “deleted” is “I,” and the file is the object, the thing deleted, so the pronoun is an object and can go. In “the service that handles auth,” the subject of “handles” is the service itself; the pronoun is the subject of its own clause, so it cannot be dropped. “The service handles auth” is a different sentence, not a reduced relative.

A quick way to feel the line: if the noun does the action, keep the pronoun; if something else does the action to the noun, you may drop it. “The engineer who wrote the script” keeps “who” (the engineer wrote it, subject). “The script [that] the engineer wrote” can drop “that” (the engineer wrote the script, so the script is the object). And remember the rule from earlier: this dropping is a defining-clause privilege only. A non-defining aside, “the script, which the engineer wrote,” can never lose its “which,” even though the pronoun is the object there too, because the comma-marked aside needs the pronoun to stay anchored.

Reduced relatives and participle clauses

Once you can drop the pronoun, the next move is to drop even more and compress the whole clause into a participle phrase. This is the reduced relative, and it is everywhere in tight, professional writing, because it says the same thing in fewer words. There are two shapes, one for ongoing or active meaning and one for completed or passive meaning.

The first shape uses the present participle, the “-ing” form, and replaces an active relative clause. “The service that handles auth” reduces to “the service handling auth.” “The team that owns this repo” reduces to “the team owning this repo.” You delete the relative pronoun and the form of “be” or the auxiliary, turn the verb into its “-ing” form, and the meaning survives intact. It works when the relative clause was active and the noun was doing the action.

The second shape uses the past participle and replaces a passive relative clause. “The bug that was fixed yesterday” reduces to “the bug fixed yesterday.” “The config that is defined above” reduces to “the config defined above.” “The pull request that was approved by Sam” reduces to “the pull request approved by Sam.” Here the noun is on the receiving end of the action, the clause was passive, and the past participle carries that passive sense by itself. The code block below sets the full clauses and their reduced forms side by side so the deletion pattern is visible.

The two patterns map cleanly onto the active and passive distinction: an “-ing” reduction keeps the active sense (the thing acts), a past-participle reduction keeps the passive sense (the thing is acted on). The compression is not just shorter; it reads as more assured, because the writer trusts the participle to carry the relationship without a full clause to spell it out. Reduced relatives lean on the participle system, which is developed further alongside the tense work in advanced tense and aspect.

Where the preposition goes: stranding versus pied-piping

When a relative clause contains a preposition, English gives you two places to put it, and the choice is almost purely one of register. Take “the tool” plus “I work with that tool.” You can leave the preposition at the end of the clause, where it falls naturally in speech: “the tool I work with” or “the tool that I work with.” Or you can move the preposition to the front, dragging the relative pronoun along with it: “the tool with which I work.” Both are correct. They differ in formality, not in meaning.

Leaving the preposition at the end is called preposition stranding, because the preposition is left stranded, separated from the noun it relates to. It is the default in everyday and spoken English, and it sounds natural in almost all writing too: “the database we migrated from,” “the colleague I reported to,” “the framework she built it on.” The old schoolroom warning against ending a sentence with a preposition has no basis in how English actually works, and following it slavishly produces stilted prose.

Moving the preposition to the front, taking the pronoun with it, is called pied-piping, a vivid name borrowed from the piper who led the rats away: the preposition leads the pronoun along behind it. “The tool with which I work,” “the database from which we migrated,” “the colleague to whom I reported.” Pied-piping is more formal and can sound stiff in casual contexts, but it is the right choice in formal writing, in legal or technical precision, and whenever stranding would leave the preposition awkwardly far from its pronoun. Note that pied-piping forces the relative pronoun to be “which” or “whom,” never “that”: you can write “the tool that I work with” but never “the tool with that I work.”

The table below pairs the two placements across registers so you can match the form to the situation.

Read down the two columns and you can hear the register climb: the left side is how people talk and write most of the time, the right side is what you reach for when the document needs to sound formal or when the preposition would otherwise dangle far from its anchor. Neither is more correct; they are tuned to different settings. The deeper behavior of which preposition pairs with which verb is a separate skill, treated in dependent prepositions.

The wider relatives: sentential which, whose, and where, when, why

The relatives we have seen all attach to a noun. Three further constructions widen the system, and each fills a gap the basic clause cannot.

The first is sentential or connective “which,” where “which” refers not to a single noun but to the whole clause before it. “The build failed, which blocked the release.” Here “which” does not stand for any one word; it stands for the entire fact that the build failed, and the clause comments on that fact. This is always non-defining, so it always takes a comma, and it always uses “which,” never “that.” It is how you bolt a consequence or a comment onto a complete statement: “She rewrote the parser overnight, which impressed everyone,” “We shipped without tests, which we regretted.” The trap is to write it as a separate sentence starting with “Which,” which is a fragment; sentential “which” must hang off the previous clause with a comma.

The second is “whose,” the possessive relative, which covers both people and things despite looking like it belongs only to people. “The engineer whose code we deployed” (a person’s code) and “the service whose latency spiked” (a thing’s latency) are both correct. English has no separate possessive relative for things, so “whose” does the work for both, and the awkward-sounding alternatives (“the service the latency of which spiked”) are usually best avoided. Use “whose” freely for non-human possessors; it is standard, not a slip.

The third group is the relative adverbs “where,” “when,” and “why,” which introduce relative clauses about place, time, and reason. “The repo where we keep the configs” (place), “the sprint when the outage happened” (time), “the reason why the deploy failed” (reason). Each replaces a clunkier preposition-plus-which form: “where” stands in for “in which,” “when” for “during which,” “why” for “for which.” So “the repo where we keep the configs” is a smoother “the repo in which we keep the configs,” and “the day when it broke” beats “the day on which it broke” in all but the most formal prose. These relative adverbs are the natural, idiomatic choice for place, time, and reason, and reaching for “in which” or “on which” instead can tip your writing into stiffness, a balance covered further in advanced punctuation and word order and information structure.

Mental Model: the comma is a switch, not decoration

Here is the reframe that makes the whole topic click, and it corrects the model almost every learner carries in. The wrong model is that commas around a relative clause are a matter of taste or rhythm, sprinkled in where the sentence seems to want a pause, the way you might pause for breath when reading aloud. Under that model, adding or removing a comma is a light edit, a polish, something a reviewer can do without changing what you said. That model is why the on-call sentence from the opening got silently rewritten.

The better model is that the comma is a logical switch with two settings, and flipping it changes the meaning, not the music. Setting one, no comma, tells the reader: this clause is part of identifying the noun, do not remove it, the noun is incomplete without it. Setting two, comma, tells the reader: the noun is already fully identified, this clause is an extra remark, you could lift it out and the core statement would stand. These are not two shades of the same sentence; they are two different claims. “The engineers who were on call missed it” blames a subset. “The engineers, who were on call, missed it” blames all of them and notes their status in passing. The comma is the only thing that distinguishes a finger pointed at a few from a finger pointed at everyone.

Once you hold this, the rest of the page reorganizes around it. The ban on “that” in non-defining clauses, the ban on dropping the pronoun in non-defining clauses, the pairing of “that” with no comma and “which” with a comma in American style: all of these are downstream of the one fact that the comma sets the job, and the job dictates which words are legal. You stop asking “does this need a comma for flow?” and start asking “is this clause identifying the noun, or remarking on it?” The answer to that question, not the rhythm of the sentence, decides the comma.

Common mistakes that survive into C1

The first mistake is the comma that flips the meaning, the error from the opening. A writer adds commas around a defining clause, turning a filter into an aside, or strips the commas from a non-defining clause, turning an aside into a filter. “The customers who complained got refunds” (only the complainers) is not “the customers, who complained, got refunds” (all of them, and they all complained). The fix is the mental model: decide whether the clause identifies or remarks, then let that decide the comma, never the rhythm.

The second mistake is using “which” for a person. “The engineer which wrote the script” is wrong; people take “who” (or, in a defining clause, “that”), and “which” is for things. This one is easy to hear once you listen for it, but it slips out under time pressure, especially in a long sentence where the person-noun is far from the relative pronoun. Scan your relative clauses and check that every “which” points at a thing.

The third mistake is the resumptive pronoun: leaving a redundant “it,” “them,” or “him” inside the relative clause that the relative pronoun has already covered. “The bug that I fixed it was in the parser” is wrong, because “that” already stands for the bug, so the extra “it” doubles the object. The correct form is “the bug that I fixed was in the parser.” This error is common among speakers of languages that keep a pronoun in the relative clause, and it survives into advanced English because the meaning is still clear. The fix is to remember that the relative pronoun is the object; you do not get to name the object twice.

The fourth mistake is using “that” in a non-defining clause: “Our lead engineer, that joined last year, is leaving.” The comma marks an aside, and an aside never takes “that.” The fix is automatic once the rule is internalized: if there is a comma, the word is “who” or “which,” never “that.” Pair this with the third common slip on the American side, writing defining “which” where a strict style guide wants “that,” and you have the two halves of the that-which problem: “that” intrudes where it is banned (after a comma), and “which” intrudes where some editors ban it (a defining clause about a thing).

The fifth mistake is over-reducing. A writer compresses one relative clause too many and produces a stack of participles the reader cannot parse: “the alert triggered firing paging the team owning the service handling auth.” Each reduction was legal; together they are a fog. The fix is the earlier guidance: reduce when it tightens and the attachment stays obvious, and keep the full clause the moment a participle could attach to more than one noun.

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