How do I choose between “that” and “which” in a relative clause?
Quick answer
Ask whether the relative clause identifies which person or thing you mean. If it does, it is restrictive: don’t set it off with commas, and “that” is commonly used in formal American writing. If it merely adds information, set it off with commas; “which” is commonly used for this purpose. These are useful conventions, not universal rules for every style guide or dialect.
The question, the details, the next step.
What’s the practical difference between “that” and “which” in a relative clause?
Start with the clause’s job, not the word choice. A restrictive clause narrows down the noun: it tells the reader which particular person or thing you mean. It normally has no commas. A supplementary clause adds information about a noun that is already identified, so it can be removed without changing which person or thing is meant; commas mark it as an aside.
Purdue OWL advises that “that” is used only in restrictive clauses and that “which” is typically preferred in nonrestrictive clauses. It also notes variation in usage, so treat this as a practical convention, not a rule that every style guide or dialect follows identically. [S1]
Compare these:
- Selecting particular files: “The files that are backed up can be deleted.” The clause identifies which files are meant: the backed-up ones. There are no commas.
- Adding information about identified files: “The files, which are backed up, can be deleted.” The clause adds information about the files already under discussion. The commas set it off as supplementary information.
The comma changes the reader’s interpretation, not just the rhythm. If the intended meaning is unclear, revise the sentence rather than relying on punctuation alone.
How can I test whether a clause is essential or just extra information?
Try removing the clause, then ask whether the remaining words still identify the same thing. If removing it changes which item you mean, the clause is probably restrictive. If the noun still points to the same item and the clause merely adds a comment, it is probably supplementary.
For example:
- “Send the draft that includes the budget.” The clause picks out one draft from possible drafts. Without it—“Send the draft”—the reader may not know which one.
- “Send the draft, which includes the budget.” The clause adds a detail about an already identified draft. Without it, the request still refers to that draft.
The test is about intended meaning, so context matters. A sentence can be ambiguous if readers cannot tell whether you are selecting one item or describing all the items under discussion.
Can you give me a reusable comparison I can use when editing?
Use these paired examples as a quick reference. Each pair keeps the noun similar while changing whether the clause selects an item or adds a comment:
- Selecting particular reports: “The reports that need revision are in this folder.” The clause identifies which reports. There are no commas.
- Adding a comment about the reports: “The reports, which need revision, are in this folder.” The clause is presented as extra information about the reports. The commas set it off.
- Selecting particular seats: “Seats that are near the stage cost more.” The clause identifies the seats whose price is being described.
- Adding a comment about identified seats: “Those seats, which are near the stage, cost more.” The clause adds a detail about seats already pointed out by “those.”
These sentences illustrate the distinction; they do not establish what a particular organization’s style guide requires. If you are editing for publication, apply the relevant guide consistently, especially if it permits a different convention.
What should I do when both versions seem possible, or my style guide differs?
First decide what the reader should understand: are you identifying a subset, or adding information about an already identified noun? Then punctuate to show that meaning. If your style guide sets a different preference for “that” and “which,” follow it while keeping the commas consistent with the intended meaning. Purdue OWL’s guidance is useful, but it does not establish a universal rule for all dialects or style guides. [S1]
If the distinction still feels hard to follow, rewrite. For instance, instead of “The files, which are backed up, can be deleted,” you could write “You can delete the backed-up files” if you mean only those files, or “The files are backed up, and you can delete them” if you mean all the files under discussion. A rewrite makes the intended scope more explicit.
A practical checklist
Identify the noun the relative clause describes.
Ask whether the clause selects which person or thing you mean.
Leave commas out for a restrictive clause; use commas around a supplementary clause.
Use “that” for restrictive clauses and “which” for supplementary clauses if that matches your style guide.
If readers could infer the wrong scope, rewrite the sentence more explicitly.
A few more useful answers
Does “which” always need a comma?
No. The key distinction is whether the clause is restrictive or supplementary, not the word alone. Purdue OWL describes “which” as typically preferred for nonrestrictive clauses, while acknowledging variation in usage. [S1]
Can I use “that” in a supplementary clause?
The cited Purdue OWL guidance says “that” is used only in restrictive clauses. Conventions vary, so follow the style guide you are writing for rather than treating that guidance as universal. [S1]
What if my sentence could mean either thing?
Decide whether the clause identifies a subset or adds a detail, then revise the sentence to make that scope clear. Rewording is often clearer than asking punctuation to resolve an unclear meaning.
Sources
References used for this guide.
- [S1] Introduction to Defining Clauses — Purdue OWL owl.purdue.edu · Checked