Most people learn that names and titles (of books, magazines, etc.) should be automatically surrounded by commas, but in fact that’s not quite true. It actually depends on the circumstances, and having a comma vs. no comma can drastically change the meaning of a sentence.

This is a rule that is best discussed through examples, so here goes. Consider the following sentence:

With Commas: Last night, James and his friend, Peter, went to see a movie.

The commas around the word Peter tell us that we are talking about one specific friend, and that the friend is named Peter. Taken out of context, it can also imply that James has only one friend, and that the friend is named Peter.

Now let’s look at the sentence without commas:

No Commas: Last night, James and his friend Peter went to see a movie.

This sentence means that Peter is one of multiple friends that James has, and that James went to the movies last night with the friend named Peter.

One more example:

With Commas: Michael Pollan’s book, In Defense of Food, has caused many people to examine closely the ingredients of the foods they eat.

The commas indicate that we are talking about one specific book by Michael Pollan: In Defense of Food.

No Commas: Michael Pollan’s book In Defense of Food has caused many people to examine closely the ingredients of the foods they eat.

This version means that Michael Pollan has written multiple books and that we are talking about the one that happens to be entitled In Defense of Food. Make sense? This rule can be tricky when it appears because it requires you to actually thinking about the meaning — and the implication — of a sentence instead of just automatically applying a rule. Questions covering it don’t appear on every on ACT, but they’ll show up often enough to make the rule worth your while to learn.