JS Tip 29: From the Writing Workshops: Using Commas for Clarity
Ian Kloville of Wells Fargo asked about using commas to increase clarity.
Among other things, commas can help you separate “nice-to-know” information from “need-to-know” information.
Let’s suppose you’ve got an uncle who loves to fish. He’s also a rocket scientist. Notice the difference between these two sentences and their use of commas:
a. My uncle, who is a rocket scientist, loves to fish.
b. My uncle who is a rocket scientist loves to fish.
In sentence “a,” you’re adding information. You have no other uncles, but you thought you’d just add that he’s a rocket scientist. (Hey. It’s impressive.) It’s “nice to know” information, so you use commas.
In sentence “b,” you’re differentiating your uncle the rocket scientist from your other uncles (the butcher, the baker, and the dentist). You add “who is a rocket scientist” so your reader will know which uncle you’re talking about. It’s “need to know” information, so you don’t use commas.
Use two commas (or two dashes or two parentheses) to separate unnecessary (nice-to-know) information in your sentences.
Our vacation—to Bermuda—was a disaster. (Nice-to-know information. You could have said “Our vacation was a disaster.” Use the dashes to separate the information.)
Our vacation to Bermuda was a disaster. (Need-to-know information. You’re differentiating your vacation to Bermuda from your vacations to Europe and Thule. No dashes.)