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.
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:
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.
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!