[Editor’s note: This post refers and links to the 17th edition of CMOS.]
Spotlight on CMOS 6.27
If you’ve ever had to learn how to use commas with relative clauses—especially clauses introduced by which or that—you may have also encountered the word restrictive and its opposite, nonrestrictive. What do those two words mean, and what do they have to do with commas?
Let’s start with relative clauses.
Relative Clauses
According to CMOS 5.226, “A relative clause is a subordinate clause that is introduced by a relative pronoun.” Common relative pronouns include that, which, and who (or whom or whose); in some cases, a relative pronoun can be omitted without loss of meaning.
A relative clause is said to be subordinate because it’s dependent on the main, independent clause that forms the basis of a complete sentence. (A relative clause cannot stand on its own.) Usually, a relative clause follows a noun that it in turn modifies, telling us something about that noun.
In the following examples, the relative clauses appear in bold type, and red has been added to single out the relative pronouns (except in the third example, where whom has been omitted):
The manuscript that the author submitted to the publisher was well formatted.
The study was limited to US citizens who have never been to Canada.
The drivers we hire to make deliveries must have good driving records.
The author’s final manuscript, which was well formatted, was submitted to the publisher on time.
Boris Pasternak, whose most famous creation was a doctor, wrote what is probably the best novel about the Russian Revolution.
In the first example, the clause “that the author submitted to the publisher” tells us something about the noun “manuscript.” The relative clauses in the other examples modify “US citizens,” “the drivers,” “the author’s final manuscript,” and the author Boris Pasternak.
Note the absence of commas in the first three examples. Each of those sentences features a restrictive relative clause. In the fourth and fifth examples, which include commas, the relative clauses are nonrestrictive.*
Restrictive = Defining and Essential
A relative clause is said to be restrictive if it is essential to understanding the intended meaning of a sentence. This usually means that it defines or otherwise identifies a noun that it modifies, thereby restricting its meaning to something specific.
When a relative clause is restrictive, it isn’t set off from the rest of the sentence by commas. For example, it would be wrong to use commas to set off the relative clause in the following sentence:
The manuscript that the author submitted to the publisher was well formatted.
not
The manuscript, that the author submitted to the publisher, was well formatted.
We know that clause is restrictive because it can’t be removed without losing essential information:
The manuscript was well formatted.
Okay, but which one? The relative clause in the original sentence told us which manuscript was well formatted—the one that the author submitted to the publisher. Remove that clause and we no longer know.
That’s the test: A clause is restrictive if it can’t be removed without obscuring the identity of the noun it modifies.
To take another example from the previous section, what does the sentence about the study (the second example) mean? Was the study limited to US citizens? No. It was limited to US citizens who have never been to Canada. The relative clause provides essential information and is therefore restrictive.
Nonrestrictive = Parenthetical and Unnecessary
A relative clause is said to be nonrestrictive if the information it provides is parenthetical. Such information may be interesting or even valuable, but it isn’t needed to identify the noun modified by the clause.
Nonrestrictive relative clauses are set off from the rest of the sentence by commas:
The author’s final manuscript, which was well formatted, was submitted to the publisher on time.
not
The author’s final manuscript which was well formatted was submitted to the publisher on time.
We know that the relative clause in that example is nonrestrictive because it can be removed without losing essential information:
The author’s final manuscript was submitted to the publisher on time.
Which manuscript? The author’s final manuscript. The phrase author’s final restricts the meaning of the word manuscript—much like a restrictive relative clause would do. The fact that the manuscript was well formatted isn’t something we need to know, so the which clause is nonrestrictive.
You could even put the clause in parentheses instead of using commas:
The author’s final manuscript (which was well formatted) was submitted to the publisher on time.
You can’t do that with restrictive relative clauses.
Which vs. That
In the previous two sections, you may have noticed how the word that introduced a restrictive relative clause, whereas which introduced a nonrestrictive clause.
In contemporary English, the word that never introduces a nonrestrictive relative clause. This, in turn, means that that clauses are never set off by commas:
Novels that feature happy endings shouldn’t all be dismissed as unserious.
not
Novels, that feature happy endings, shouldn’t all be dismissed as unserious.
Which, on the other hand, can go either way. But in American English, which is normally reserved for nonrestrictive clauses, which are always set off by commas:†
Mystery novels, which usually feature a serious crime, became popular in the 1800s.
not
Mystery novels which usually feature a serious crime became popular in the 1800s.
Many writers, however—notably in British English—use which with restrictive relative clauses:
Novels which feature happy endings shouldn’t all be dismissed as unserious.
A copyeditor might want to change that which to that. But even if you’re working with American English, it’s usually safest to get the author’s or publisher’s approval before changing every restrictive which to that.
Beyond Relative Clauses
This post focuses on relative clauses, but the terms restrictive and nonrestrictive can also apply to other contexts.
For example, a “Jr.” following a name—as in Sammy Davis Jr.—could be called restrictive (it tells us which Sammy Davis is meant, father or son), as E. B. White of Strunk & White fame concluded in the 1970s as a rationale for dropping the traditional comma.
And if you had two sisters and referred to “my sister Rita,” then Rita would be restrictive, telling us which sister. If Rita were your only sister, strict usage would require “my sister, Rita”—in which Rita would now be considered nonrestrictive. That rule, however, isn’t set in stone.
But when it comes to using commas with relative clauses, it’s generally best to follow the rules, however restrictive they might be.
* And did you notice “which include commas”—a nonrestrictive relative clause—in that footnoted sentence?
† Another nonrestrictive relative clause: “which are set off by commas.” In this case, however, only one comma is needed, because the clause ends in a colon.
Which? by MichaelJBerlin / Adobe Stock.
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