Q. What is a “restrictive” appositive? I have read chapters 5 and 6, but I do not understand the difference between the two types of examples, distinguishing the use of commas by this term. Please advise . . . my client wants to know the “why” behind my editorial source.

A. “Restrictive” appositives cannot be removed from a sentence without obscuring the identity of the word or phrase that the appositive is intended to identify. Take the following two sentences:

My cat Philby is fat. [I have two cats.]
My cat, Philby, is fat. [I have one cat.]

Note the similarities; if you study the two sentences closely, you will see that they are in fact identical—except for the commas. The problem is that if you take “Philby” away from the first sentence—whereas you know I have two cats—you’d have no way of knowing, assuming no additional context, which of my two cats I was talking about. If you take “Philby” away from the second sentence—knowing that I have just one cat—you’d still know it’s Philby that has the weight problem. The commas serve to set off what is essentially ancillary—or nonrestrictive—information.