One use of the comma is that it sets off information that may be interesting but is not necessary for the reader to understand the sentence. That information set off in commas is called “non-essential.”Here are a few examples:
1. My cousin, Juliet, lives in Seattle.
By using commas around her name, you are telling the reader you have only one cousin and her name happens to be Juliet. If you remove the commas and her name, your readers will understand that you have only one cousin. If you keep her name but remove the commas, you are telling the reader you have more than this one cousin. How many more? We don’t know, but Juliet is not the only one.
2. Let’s eat, Eddie, before we pitch our tents.
As written, this sentence is what is called “direct address.” We are speaking directly to Eddie and saying we want to have a meal before we settle in for the night on our camping trip. If you take those commas out, suddenly this becomes the cannibal camping trip, and Eddie is getting very, very nervous.
3. (X-rated) Sam helped his brother, Jack, off his horse.
Speak the sentence without the commas. See how important commas can be?