Which of these punctuation marks is used to set off a nonessential element in a sentence?

PUNCTUATION: CONSTRUCTING SENTENCES

Independent Clauses

A phrase that contains both a noun and a verb and can stand alone as a sentence.

Internal Comma

A comma contained within a clause, as opposed to a comma that appears before or after the clause.

Nonessential Element

A word, phrase, or clause that modifies but does not limit or change the essential meaning of a sentence.

Transitional Words or Phrases

Words or phrases that link independent clauses. Common transitional words and phrases include:

• accordingly
• afterwards
• again
• besides
• consequently
• doubtless
• eventually
• evidently
• furthermore
• however
• moreover
• nevertheless
• otherwise
• perhaps
• therefore
• for example
• for instance
• in addition

RULES

Pay attention to the way the following sentences are constructed; each sentence illustrates the rule it describes.

Semicolon or Comma

• Use a comma to separate two independent clauses that are joined together by and, but, or, for, nor, yet or so; if those words are omitted, use a semicolon. 
• The exception: use a semicolon if one or both clauses contain an internal comma (that is, a comma within the clause itself) where a misreading may occur if a comma is used. 

Dash

• Use a dash instead of a comma if you want to set off a nonessential element that needs special emphasis—but use it sparingly and for deliberate effect.
• If a nonessential element contains an internal comma—again, a comma within the element itself—use dashes instead of commas to set the element off. 

Parentheses

• Use parentheses to set off explanatory material (words that provide additional explanation or information) that is not necessary to the meaning of the sentence. 
• If you want to set off a nonessential element (words, phrases, or clauses that are not necessary to the structure or meaning of a sentence) but dashes would provide too much emphasis, and commas might be confusing or inappropriate, use parentheses. 

Colon or Semicolon

• With independent clauses, use a colon if the second clause explains the first: the second clause illustrates or provides specific information about the first clause.
• If the second clause does not explain the first clause, use a semicolon; using a colon in the sentence you are now reading would be wrong.
• Use a semicolon if the clauses are linked by transitional words or phrases; for example, phrases like however, besides, therefore, for example, for instance, and in addition are all transitional.
• If you’re unsure whether to use a colon or semicolon, your best bet would be to treat each clause as a separate sentence and use a period after each one.

Although this guide has been designed to address common punctuation problems, it is by no means exhaustive. If your specific concerns have not been addressed we encourage you to ask a tutor, check a reference manual, or see your professor.  

Commas wield a lot of power. They can, for instance, determine whether a piece of descriptive language is essential, or not essential, to the meaning of a sentence.1 

When Commas Should Be Omitted

Some sentences only make sense when the description is treated as essential. In sentences like these, always omit commas. 

People who do not learn from their mistakes are bound to fail.

The descriptive element in this sentence—“who do not learn from their mistakes”—is essential, because it is critical to the meaning of the sentence, which emphasizes the importance of learning from one’s mistakes. If it is removed from the sentence, the result—People are bound to fail—is nonsensical, or at least radically pessimistic!

When Commas Should Be Used

Other sentences only make sense when the description is treated as nonessential. In these sentences, always place commas around the descriptive language.

American teenagers, who have no memories of the 1990s, see the world very differently from their parents.

The descriptive element in this sentence—“who have no memories of the 1990s”—is nonessential, because it can be removed without changing the central meaning of the sentence. If it is removed, the resulting sentence—American teenagers see the world very differently from their parents—makes the same claim as the original. In fact, removing the commas makes the sentence illogical—American teenagers who have no memories of the 1990s see the world very differently from their parents—because today no American teenager can remember the 1990s.

When Commas May Be Used or Omitted

Finally, some sentences make sense either way–when the descriptive language is treated as essential and when it is treated as nonessential. In sentences like the following one, omitting the comma makes the description essential; adding a comma makes it nonessential.

Essential

Barbara always wakes up at six a.m. when her neighbor’s dog barks.

The phrase “when her neighbor’s dog barks” is essential to the meaning of the sentence. In other words, Barbara wakes up at six only on the days when her neighbor’s dog barks–presumably, the barking dog wakes her up. If you remove this description, the sentence—Barbara always wakes up at six—means something totally different, as you can see in the version below:   

Nonessential

Barbara always wakes up at six a.m., when her neighbor’s dog barks.

The comma that precedes “when her neighbor’s dog barks” makes the description nonessential. That is, the fact that the dog barks is an extra piece of information about what happens at six in the morning—not the thing that causes Barbara to wake up.

As you can see, commas do a lot more than tell you where to pause and catch your breath when you read a sentence. They can radically change the meaning of a sentence or render it nonsensical or absurd. To decide whether or not to use commas with a description, just ask yourself, Is the description essential to the meaning of my sentence? If the answer is yes, do not use commas. If the answer is no, then do use them. 

Note

1Essential and nonessential descriptions are also known, respectively, as “restrictive” and “nonrestrictive.”

Think you understand commas with essential and nonessential elements? Test your knowledge with our quiz! 

Filed Under: grammar, punctuation, writing tips

Barney Latimer

As senior editor of MLA publications, Barney Latimer has copyedited PMLA articles for more than ten years. He holds an MA in English from New York University. He has taught high school and college classes in writing and literary analysis, as well as seminars in poetry writing at several nonprofit organizations that serve New Yorkers with mental illness.

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