Setting Off Extra Info with Commas

by Michael Moreno

Are you confused about when to use commas to add extra information into the middle of a sentence?  The secret is to determine whether or not that information is actually extra information or if it’s necessary information for the sentence to make sense. The grammar handbooks rattle off stuff about nonessential and essential phrases and clauses, but here we’ll keep it simple: Is it extra, or is it necessary?

Take, for instance, the following sentence:

• Matthew, who lives in Memphis, was the only Tennessean at the event.

Readers would understand it, and no meaning would be lost, if the sentence simply read:

• Matthew was the only Tennessean at the event. 

Commas indicate to readers that the words in between them add extra information. It might be useful to picture it this way:

• Matthew, who lives in Memphis, was the only Tennessean at the event. 

On the other hand, consider this example:

• Karen has three brothers who live in three cities. Her brother who lives in Chicago came to see her over the weekend.

In this case, it’s critically important that “who lives in Chicago” be included in the sentence for readers to understand who visited her. If we’d set off “who lives in Chicago” with commas, it would indicate this to readers:

• Karen has three brothers who live in three cities. Her brother, who lives in Chicago, came to see her over the weekend.

The commas indicate that the information can be deleted without changing the meaning of the sentence. But, if you deleted the information between the commas in the above sentence, readers would wonder which brother visited her.

Now, let’s say that Karen has only one brother.

• Karen’s brother, who lives in Chicago, came to visit.

In this case, because she has only one brother, “who lives in Chicago” is not needed to differentiate between other brothers, because they do not exist. So, the commas are OK! We could delete the information between them, and the meaning of the sentence would not change:

•  Karen’s brother, who lives in Chicago, came to visit.

Lastly, the word “that” usually indicates that commas are not needed. For example:

• The book that I’m reading now is fascinating.

If we crossed out “that I’m reading now,” the sentence would be confusing. Readers would wonder which book we’re talking about. Visualize it this way:

• The book that I’m reading now is fascinating.

In sum: use commas to indicate when extra information has been added. Don’t use commas to set off information that is critically important for understanding. Follow these guidelines and your sentences will be clearer and make more sense to any reader, including your professor!