Another Case of a Tricky Comma


My Facebook friend Mike recently kicked off a discussion about a comma job that can confuse even the most punctuation-savvy person. Consider these passages.

The class was assigned to read Herman Melville's most famous work. Students were told to buy the book, Moby Dick, before Tuesday.

The class was assigned to read the book Moby Dick and watch the film Casablanca.

Both passages contain the phrase "the book Moby Dick," but the first puts commas around the title while the second one does not. And they're both correct. How is that possible?

There are a couple ways to parse this, but the simplest is captured by the terms "restrictive" and "nonrestrictive" information. When a word or phrase is restrictive, it narrows down the scope of things you might be talking about.

For example, the first sentence of this blog post mentions my Facebook friend Mike. If I only had one Facebook friend, the name Mike would in no way narrow down who I was talking about. But because I have more than one friend, the name Mike literally restricts the scope of the noun "friend." It narrows down the field to a specific person.

The rule is: restrictive information takes no commas, like the name Mike in my first sentence. Nonrestrictive information, which Mike would be if I had only one friend, is set off with commas. I

In our first passage, we already specified which one book we were talking about: Melville's most famous. So the title Moby Dick doesn't narrow down the range of things we might mean by "the book." But in our second passage, the reader wouldn't know what book you were talking about without the title. So that's restrictive information and takes no commas.

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