In English grammar, modifying clauses often give readers crucial details about the people, places, and things in a sentence. Most of these modifiers are relative clauses (also called adjective clauses)—dependent clauses that begin with words like who, whom, whose, which, or that.
To punctuate and construct these clauses correctly, you must determine whether the information is essential (restrictive) or extra (nonrestrictive).
What Is a Restrictive Clause?
A restrictive clause (also called an essential or defining clause) provides information necessary to identify the specific noun it modifies. It limits or restricts the meaning of the noun.
If you remove a restrictive clause, the basic meaning of the sentence changes significantly or becomes vague, because the reader will no longer know which specific person or item you are talking about.
Punctuation Rule
- Do not use commas to set off a restrictive clause. Because the information is integral to the sentence’s core identity, it should not be separated from the noun.
Examples
- Example: Drivers who run red lights endanger pedestrians.
- Meaning: The sentence does not claim that all drivers endanger pedestrians—only the specific subset of drivers who run red lights.
- Example: The book that you lent me was fascinating.
- Meaning: The clause that you lent me specifies exactly which book is being discussed among all possible books.
What Is a Nonrestrictive Clause?
A nonrestrictive clause (also called a nonessential or non-defining clause) provides supplementary, “bonus” information about a noun that is already clearly identified.
If you remove a nonrestrictive clause, the remaining sentence still clearly identifies the subject and maintains its core message.
Punctuation Rule
- Always set off a nonrestrictive clause with commas.
- If the clause appears in the middle of a sentence, place a comma before it and a comma after it. If it ends the sentence, place a comma before it and end with a period.
- Note: Em dashes (
—) or parentheses (( )) can also be used instead of commas to set off a nonrestrictive clause when you want to emphasize an interruption or create a casual aside.
Examples
- Example: My oldest sister, who lives in Chicago, is visiting this weekend.
- Meaning: Because I have only one oldest sister, her identity is already established. The detail about where she lives is just interesting background information.
- Example: The Eiffel Tower, which was completed in 1889, attracts millions of visitors each year.
- Meaning: The proper noun The Eiffel Tower is already fully identified. The year of completion is parenthetical detail.
How Punctuation Changes Meaning: Paired Examples
A single pair of commas can completely alter the meaning of a sentence.
The Student Example
- Restrictive: The students who studied passed the exam.
- Meaning: Only the specific students who studied passed; the students who did not study failed.
- Nonrestrictive: The students, who studied, passed the exam.
- Meaning: Every student in the group studied, and all of them passed. The fact that they studied is just an added descriptive note about the whole class.
The Painting Example
- Restrictive: Please return the painting that hangs in the lobby.
- Meaning: There are multiple paintings in the building; return the specific one located in the lobby.
- Nonrestrictive: Please return the painting, which hangs in the lobby.
- Meaning: There is only one painting being discussed. As an aside, it happens to hang in the lobby.
Choosing the Right Relative Pronoun
Relative clauses are introduced by relative pronouns. Selecting the correct pronoun depends on whether the clause refers to a person or an object, and whether it is restrictive or nonrestrictive.
| Pronoun | Refers To | Restrictive? | Nonrestrictive? | Usage Notes |
|---|---|---|---|---|
| That | Things, concepts, animals | Yes | No | Standard English does not use that for nonrestrictive clauses. |
| Which | Things, concepts, animals | Yes (qualified) | Yes | Standard in nonrestrictive clauses (with commas). In formal American English, that is preferred for restrictive clauses, though which is frequently used restrictively in British English and informal writing. |
| Who | People (subject) | Yes | Yes | Used when the pronoun acts as the subject of the clause (the person who called). |
| Whom | People (object) | Yes | Yes | Used when the pronoun acts as the object of a verb or preposition (the mentor whom I consulted). |
| Whose | People, things (possession) | Yes | Yes | Shows ownership (the author whose book won an award). |
5. Summary Comparison
| Feature | Restrictive (Essential) | Nonrestrictive (Nonessential) |
|---|---|---|
| Function | Identifies or narrows down the noun. | Adds incidental, non-identifying detail. |
| Necessity | Essential to identify which one. | Can be removed without losing identity. |
| Punctuation | No commas. | Set off by commas (or dashes/parentheses). |
| Pronouns for Things | that (preferred in US) or which | which |
| Pronouns for People | who, whom, whose | who, whom, whose |
Practical Identification Tips & Tests
When deciding whether to use commas, apply these two simple tests:
1. The Deletion Test
Read the sentence without the clause.
- If the sentence loses its primary meaning or leaves the reader asking “Which one?”, the clause is restrictive (no commas).
- If the core statement remains intact and the subject is still unambiguous, the clause is nonrestrictive (use commas).
2. The “Which One?” Question
Ask yourself: Does the reader already know exactly which noun I am referring to before reading this clause?
- If the noun is a proper noun (e.g., Albert Einstein, Tokyo, Mount Everest) or a unique family member (e.g., my mother), the clause is almost always nonrestrictive because the noun is already identified.
Common Mistakes to Avoid
Mistake 1: Placing Commas Around Restrictive Clauses
- Incorrect: The car, that won the race, was custom-built.
- Correct: The car that won the race was custom-built.
- Why: The clause that won the race identifies which car is meant. Commas incorrectly signal that the information is optional.
Mistake 2: Dropping the Second Comma in Mid-Sentence
- Incorrect: My laptop, which has a cracked screen still works fine.
- Correct: My laptop, which has a cracked screen, still works fine.
- Why: A nonrestrictive clause embedded in the middle of a sentence requires a pair of commas to open and close the interruption.
Mistake 3: Using “That” in a Nonrestrictive Clause
- Incorrect: Lake Superior, that borders Canada and the United States, is the largest freshwater lake by surface area.
- Correct: Lake Superior, which borders Canada and the United States, is the largest freshwater lake by surface area.
- Why: Nonrestrictive clauses describing things must be introduced by which, not that.
Practice Exercises
Determine whether the bracketed clause is restrictive or nonrestrictive, and add commas where appropriate.
- The musician [who played the violin solo] received a standing ovation.
- Maya Angelou [who was an acclaimed poet and memoirist] spoke at the presidential inauguration.
- Houses [that are built near the coastline] often require extra flood insurance.
- My favorite leather jacket [which I bought in Italy ten years ago] finally tore at the elbow.
- Employees [who complete the safety training by Friday] will receive a bonus.
Answer Key & Explanations
- Restrictive — The musician who played the violin solo received a standing ovation.
- Reason: The clause identifies which specific musician got the ovation among all the performers. No commas.
- Nonrestrictive — Maya Angelou, who was an acclaimed poet and memoirist, spoke at the presidential inauguration.
- Reason: Maya Angelou is a proper noun that uniquely identifies the person. The clause adds biographical detail. Commas required.
- Restrictive — Houses that are built near the coastline often require extra flood insurance.
- Reason: The clause specifies which houses need flood insurance (only coastal ones). No commas.
- Nonrestrictive — My favorite leather jacket, which I bought in Italy ten years ago, finally tore at the elbow.
- Reason: The phrase my favorite leather jacket already identifies the item uniquely. The purchase location is extra detail. Commas required.
- Restrictive — Employees who complete the safety training by Friday will receive a bonus.
- Reason: The clause limits the bonus solely to employees who meet the deadline. No commas.
