Start with the noun, not the comma
A relative clause describes a noun by attaching a group of words that often begins with who, which, or that. In “The notebook that has a blue cover is mine,” the clause “that has a blue cover” describes the notebook. Its most important job is to identify which notebook the speaker means. This is a defining relative clause: without it, “The notebook is mine” may leave the reader looking at several possible notebooks.
Now compare “My notebook, which has a blue cover, is mine.” The phrase “my notebook” is treated as enough to identify the object. The clause adds a detail rather than selecting one notebook from a group. This is a nondefining relative clause. The distinction is about the clause’s job in the sentence, not whether its information is interesting or useful.
Ask what happens if you remove the clause
To test a clause, temporarily take it out and ask whether readers can still tell which noun you mean. “The student who borrowed the atlas left a note” points to a particular student by describing what that student did. Remove the clause, and “The student left a note” may no longer identify the person. The clause is defining, so it has no surrounding commas.
In “Maya, who borrowed the atlas, left a note,” the name Maya already identifies the person in the intended context. Removing “who borrowed the atlas” leaves the central reference intact. The clause is nondefining, so commas mark it off. The removal test is a guide, not a mechanical rule: context matters. If two people named Maya are under discussion, a description might be needed to distinguish one from the other.
Notice how punctuation changes the meaning
Consider “The postcards that arrived yesterday are on the desk.” This suggests that the speaker means only the postcards that arrived yesterday; other postcards may be elsewhere. “The postcards, which arrived yesterday, are on the desk” presents the postcards as an identifiable set and adds their arrival time as extra information. The commas are not pauses added for style. They tell readers how the clause relates to the noun.
Use no commas around a defining clause. Set off a nondefining clause with commas if it falls in the middle of a sentence: “Our editor, who loves precise verbs, revised the opening.” If the clause ends the sentence, use a comma before it and finish with a period: “I found the draft, which I thought I had lost.” When you revise punctuation, check the intended meaning too; inserting commas can change which people or things a sentence refers to.
Choose relative words that fit the clause
Who commonly refers to people in either kind of clause: “Writers who read aloud often hear awkward rhythms” and “Lee, who reads every draft aloud, caught the repetition.” That and which can introduce defining clauses about things, though some editors prefer that for defining clauses and which for nondefining ones. In edited prose, avoid that in a nondefining clause: write “The lamp, which flickers, needs a new bulb,” not “The lamp, that flickers, needs a new bulb.”
A defining clause can sometimes omit its relative word when that word is the object: “The book that I borrowed” can become “The book I borrowed.” You cannot make the same omission in a nondefining clause: “The book, which I borrowed, has a torn cover” needs which. If a sentence sounds strained after you remove a word, restore it or recast the sentence rather than relying on punctuation to repair it.
Try the identification test while editing
Imagine a shelf holding several folders. You write, “The folder that contains the interview notes is missing.” Keep the clause unpunctuated because it identifies the missing folder. Now imagine only one folder is in view: “The folder, which contains the interview notes, is missing.” The clause supplies an additional fact about that folder. Neither version is automatically better; each serves a different situation.
Try this on your own: punctuate “My neighbor who grows roses lent me a spade.” If you have several neighbors and mean the rose grower, leave it without commas. If your reader already knows which neighbor you mean, write “My neighbor, who grows roses, lent me a spade.” Before finalizing a sentence, ask two questions: Can readers identify the noun without this clause? And do the commas match the answer?
