Last week, we talked about setting a parenthetical description off with commas in the grammatical phenomenon known as an “appositive.”

Now, we’ll discuss how to apply it to everyday descriptions introduced by the articles “a/an” and “the.”

“President Obama saw a movie” tells you about the activity, but not what movie he saw. Adding that information may require a comma, or not. It will depend on whether the article introducing “movie” is “a” or “the,” as well as whether the movie is unique and the context of the sentence.

If you said “The president saw the movie Bridesmaids,” no comma is needed, because there’s more than one movie: Its name is not parenthetical, so it should not be set off. Or, to look at it another way, “The movie Bridesmaids” is unique; there’s only one, so its name with the description “movie” is necessary to identify it.

If you said “The president saw a movie, Bridesmaids,” the comma is needed, since the emphasis is on the fact that the president saw a movie, not which movie he saw. Its name is then given as parenthetical information.

But in journalism, we frequently see constructs like “The president saw the movie, Bridesmaids.” That’s simply incorrect, a false appositive. It’s like saying “He read the book, the Bible,” which says that the Bible is the only book ever written.

There are instances, though, when “the movie, Bridesmaids,” would be correct. Here’s one example:

Saturday nights have always been movie night at the White House, and last week was no exception, even though the president was exhausted. The president watched the movie, Bridesmaids, even though he told his wife, Michelle, that he might fall asleep during it.

Since the fact that he watched a movie has already been introduced, its title becomes parenthetical, and thus an appositive. (Since Michelle is his only wife, her name is an appositive, too.)

The item under discussion doesn’t have to be unique to get the article “the” instead of “a,” of course. Introducing the star of Bridesmaids as “the actress Kristen Wiig” is more accurate grammatically than calling her “the actress, Kristen Wiig,” since there are many, many actresses. (As to whether to call her simply “actress Kristen Wiig,” stay tuned for a column on false titles.) But if an actress wasn’t very well known, she would be better introduced as “an actress, Biona Domina.” That gives a reader a pause to absorb the concept that the person about to be introduced by name is an actress, instead of putting her description and name in one breath, leading a reader to wonder, “Who?”

When you start to add more information right after something you’ve named—a victim, the name of a bill, a book, a movie, an actress—think appositive thoughts to decide whether you really need that additional information. If you need that information to understand the thing you’ve named, not merely further describe it, you do not need the commas.

Remember, too, that commas often travel in pairs. If you think you need just one, either you’re not creating an appositive, or you’re about to create a grammatical monster.

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Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years. Follow her on Twitter at @meperl.