Here’s a question one can ask at the border of grammar and logic: Reading the sentence Paul’s sister Maureen is a physician, can we know whether Paul has more than one sister? The question involves how the sentence is punctuated, and is part of a department of grammar called apposition. Some earlier posts here (Apposition, Commas: To Use or Not to Use) have already taken up the subject of apposition, but because it can be a tricky topic (involving as it does the correct use of commas) and because standard written English requires we get it right, another look at it might again be in order.
Let’s begin by defining some terms. Apposition is the placing of one element immediately after another to define or describe the first element more fully. The term element, we remember, refers in grammar to any word, phrase, or clause. In our sentence Paul’s sister Maureen is a physician, the noun Maureen has been placed immediately after the noun sister in order to say something more about Paul’s sister, in this case giving the reader her name. This adjacent element is called an appositive (a Latin term meaning put next to), and an appositive can be either restrictive or nonrestrictive.
To restrict something means to confine it, and in language, we confine something by defining it. So the somewhat intimidating term restrictive appositive means nothing more than that one element is being placed immediately after another element—without commas—in order to define the first element. To define something, another Latin term, means to put a border around it, to restrict or limit what we can think a word is referring to. So the restrictive appositive Maureen in the sentence Paul’s sister Maureen is a physician is meant to define which sister of Paul’s is, in fact, the physician. And why would the author take the trouble to define the sister by giving the reader her name? Because Paul must have at least one other sister. Restrictive appositives define and are not set off by commas.
So what would it mean to write instead Paul’s sister, Maureen, is a physician? Here, the appositive has been isolated by a pair of commas, and this transforms a restrictive appositive into a nonrestrictive one. Restrictive appositives define and nonrestrictive appositives describe; both give the reader more information (here, the name of Paul’s sister), but restrictive appositives give information that it is felt essential to know for purposes of identification, while nonrestrictive appositives give merely additional, but ultimately inessential, information. Whether one will write an appositive as restrictive or nonrestrictive will depend on the truth of the circumstances.
In the sentence Paul’s sister, Maureen, is a physician, we know then that Maureen is a nonrestrictive appositive because it has been isolated by a pair of commas, from which we could rightfully infer that Paul must have only one sister, because the writer has not felt the need to differentiate that sister from another by writing her name restrictively, without commas. Nonrestrictive elements can always be removed from the sentence without causing confusion: Paul’s sister is a physician. No confusion will result here because the truth of the matter, presumably, is that Paul has only one sister, and so the reference is clear. Nonrestrictive appositives are often given as a matter of courtesy, or to establish information that will be referred to later.
Restrictive and nonrestrictive apposition can also be seen in the difference between the relative pronouns which and that. The short answer (for the long answer, see Who, Which, That, and What) is that which can be used as both a restrictive or nonrestrictive appositive, but that can only be used as a restrictive appositive. Thus, the car which is in the garage won’t start points to the same set of circumstances as the car that is in the garage won’t start: there must be at least one other car on the scene that is not in the garage and will, presumably, start. And both of these sentences mean something different from the car, which is in the garage, won’t start. But all three of these statements do agree on one thing: it’s time to leave more on all that for another post.