Some commas, like socks, always come in pairs. Like bookends. Commas that generally come in pairs include
- commas around embedded quotations (Rule AG),
- commas around titles (Rule AF),
- commas around absolute phrases (Rule AB),
- commas around non-restrictive phrases and clauses starting with who and which (Rule U),
- commas around interpolated asides (Rule L), and
- commas around appositives (Rule J).
I found a whopper of a boo-boo today in which the writer needed a pair of commas but only used one. He used the first comma of a pair but forgot the second comma.
Here’s the sentence with the missing comma.
The five cups, believed to date from the late 17th or early 18th century were valued at $1-$1.5 million on Saturday after being brought to the TV show at a stop in Tulsa, Oklahoma. (http://tv.yahoo.com/blog/chinese-rhino-cups-set-antiques-roadshow-record–3352)
The phrase “believed to date from the late 17th or early 18th century” is a parenthetical expression. A parenthetical expression is a phrase or clause that is not part of the main point of the sentence, can be removed without making the sentence ungrammatical, and provides some supplemental description.
Parenthetical expression need to be separated from the rest of the sentence with commas (Zen Comma Rule AA). If they are embedded in the sentence, as in this example, they need a comma both before and after.
The first comma tells the reader, “Hold on. I’m about to explain something.” The second comma tells the reader, “Ok, let’s get back to the main point.” Because it signals an interruption in the sentence, it helps the reader know what is the main point and what isn’t. That pair of commas improves reader understanding.
Now, if you only have the first comma, you tell the reader, “Wait a moment while I explain something,” but you never provide the visual clue that you’re getting back to the main point. The reader may figure out what you mean, but you’re asking for a lot of work from the reader. And the harder the reader needs to work to understand you, the more likely the reader is to misunderstand.
Plus, forgetting the second comma makes you seem less credible as a writer.
In defense of this writer, however, I think this mistake was just that: a mistake. A bit later in the article, the writer demonstrates that he does, indeed, know how to use commas. Here, he uses a pair of commas to separate a non-restrictive clause:
The owner, who prefers to remain unidentified, told Asian arts expert Lark Mason he started collecting cups inexpensively in the 1970s and had no idea of the collection’s current value.
The expression “who prefers to remain unidentified” is a non-restrictive clause that describes “owner.” As noted above, non-restrictive clauses and phrases need to be separated with a pair of commas, and this one is.
Need help with commas? Get Zen Comma, an instructive reference guide on the 17 major uses and misuses of commas, available in PDF and Kindle formats. Read more about Zen Comma.