Comma Before Which: Restrictive vs. Nonrestrictive Clauses Explained

The comma before “which” might be the single most debated punctuation decision I see in student work. One writer uses it freely; another avoids it like a landmine. The confusion stems from one thing: most English learners were never explicitly taught the difference between restrictive and nonrestrictive clauses. Once you understand that distinction, the comma rule becomes obvious.

Here’s the short answer: use a comma before “which” when the clause is nonrestrictive (providing extra information). Don’t use a comma when the clause is restrictive (essential to the sentence’s meaning). You’ll breaks down both types with real examples, shows you how to test your writing, and explains why “that” and “which” aren’t interchangeable—despite what you might have learned.

Understanding Restrictive vs. Nonrestrictive Clauses

The key to this entire rule is one concept: restrictive vs. nonrestrictive. Let me define each clearly.

Restrictive Clauses

A restrictive clause 🔊 🎤 provides essential information—information that narrows down or restricts the meaning of the sentence. If you remove it, the sentence’s meaning changes or becomes incomplete.

Example 1: The car that is parked outside is mine.

The clause “that is parked outside” restricts which car we’re talking about. There might be many cars, but we’re specifying one particular car. Remove the clause and the sentence becomes vague: “The car is mine” — which car? Without the restrictive clause, we’ve lost critical meaning.

Example 2: The student who studied hardest 🔊 🎤 passed the exam.

Without “who studied hardest,” the sentence is incomplete. Which student? We need that information.

Nonrestrictive Clauses

A nonrestrictive clause 🔊 🎤 provides extra information that is NOT essential to the meaning. If you remove it, the sentence still makes complete sense.

Example 3: My car, which is red 🔊 🎤, is parked outside.

The clause “which is red” adds extra information about my car, but it’s not essential. Remove it: “My car is parked outside.” The sentence still works—we knew which car was being discussed from context.

Example 4: My brother, who is a doctor, lives in New York.

The clause “who is a doctor” tells us something extra about my brother, but it’s not essential to identify him. If I say “My brother lives in New York,” the meaning is clear.

The Comma Rule: When to Use It

Here’s the rule in its simplest form:

• Nonrestrictive clause 🔊 🎤 → Add commas around the clause.
• Restrictive clause 🔊 🎤 → No comma.

Since “which” almost always introduces nonrestrictive clauses in American English, you’ll almost always use a comma with “which.” Since “that” introduces restrictive clauses, you’ll almost never use a comma with “that.”

Clause type	Relative pronoun	Comma?	Example
Nonrestrictive	which	Yes	My car, which is red, is parked outside.
Restrictive	that	No	The car that is parked outside is mine.
Restrictive (people)	who	No	The student who studied hardest passed.
Nonrestrictive (people)	who	Yes	My brother, who is a doctor, lives in NYC.

Which vs. That: The Crucial Difference

This is where American English and British English diverge. In American English, the distinction is strict:

• “That” introduces restrictive clauses (no comma).
• “Which” introduces nonrestrictive clauses (use comma).

In British English, “which” can appear in restrictive clauses (and some writers omit the comma even for nonrestrictive “which,” though this is less common now).

American English approach:

“The book that I read last night was great.” (restrictive—use “that”)

“The book, which I read last night, was great.” (nonrestrictive—use “which”)

British English might say either:

“The book which I read last night was great.” (restrictive, using “which”)

“The book, which I read last night, was great.” (nonrestrictive)

For clarity and to follow American style guides (AP, APA, Chicago), stick to the American rule: use “that” for restrictive, “which” for nonrestrictive.

Testing Your Clauses: The Removal Method

If you’re ever unsure, use this test: Remove the clause. Does the sentence still make sense?

Example 5: The location at which we’ll arrive by 7 p.m. is the main office.

Remove “at which we’ll arrive by 7 p.m.”: “The location is the main office.” —Doesn’t make sense. We need to know which location. This is restrictive; no comma before “which.”

Example 6: The conference, which takes place next month, is in Boston.

Remove “which takes place next month”: “The conference is in Boston.” —Still makes sense. The clause is nonrestrictive; comma is correct.

Example 7: The employees who work in the sales department received bonuses.

Remove “who work in the sales department”: “The employees received bonuses.” —Incomplete. Which employees? This is restrictive; no comma with “who.”

Example 8: Sarah, who works in the sales department, received a bonus.

Remove “who works in the sales department”: “Sarah received a bonus.” —Still clear. We know who Sarah is. This is nonrestrictive; comma is correct.

Common Mistakes and How to Fix Them

✗ Incorrect: The students, that scored highest passed the exam.

✓ Correct: The students that scored highest passed the exam.

Why: Restrictive clause (essential to identify which students) uses “that,” not “which,” and no comma.

✗ Incorrect: My brother which is a doctor, lives in NYC.

✓ Correct: My brother, who is a doctor, lives in NYC.

Why: Nonrestrictive clause needs commas and prefers “who” for people. “Which” is awkward here.

✗ Incorrect: The location at which, we’ll arrive by 7 p.m. is downtown.

✓ Correct: The location at which we’ll arrive by 7 p.m. is downtown.

Why: Restrictive clause (prepositional + relative clause). No comma before “which” in restrictive contexts. Also, don’t separate a preposition from its object.

✗ Incorrect: She asked me, which book I was reading.

✓ Correct: She asked me which book I was reading.

Why: “Which” here is part of an indirect question. There’s no relative clause, so no comma is needed (and “which” isn’t a relative pronoun in this context—it’s interrogative).

Prepositional + Relative Clauses

Phrases like “at which,” “to which,” “from which,” and “by which” are common in formal writing. These are restrictive by nature (they specify a location, time, or condition), so no comma:

Example 9: The date by which the project must be completed is March 15.

Example 10: The country to which she emigrated is Canada.

Even though “which” appears, no comma is used because these clauses are restrictive.

Nonrestrictive Clauses with Other Relative Pronouns

The comma rule applies to all relative pronouns when introducing nonrestrictive clauses, not just “which.”

Pronoun	Clause type	Comma?	Example
Who	Nonrestrictive	Yes	My sister, who is a teacher, just graduated.
Whom	Nonrestrictive	Yes	The president, whom I met once, is arriving soon.
Whose	Nonrestrictive	Yes	The house, whose roof is red, belongs to my aunt.
Where	Nonrestrictive	Yes	The park, where we played as children, is now closed.
When	Nonrestrictive	Yes	The summer of 2010, when I first visited, was unforgettable.

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