Part 1: Sentence and Clause Structure
Nonrestrictive Relative Clauses
Examples
Maize, which is also known as corn, is a native plant to the North American continent.
The male part of the cob is the tassel, which releases pollen.
Henry A. Wallace, who was one of the most important corn researchers in the 1920s and 1930s, served as Vice President under Franklin D. Roosevelt.
Definition
A nonrestrictive relative clause is a type of relative clauses that expands on the meaning of a noun, noun phrase, or clause. Like all relative clauses, it starts with a relative pronoun, which can be the subject, the object, a possessive, or an object of a preposition in the relative clause.
Unlike restrictive relative clauses, nonrestrictive relative clauses add extra information about the referent noun. That is they refer to all of the noun phrase; they don’t restrict the meaning of the noun phrase:
Maize, which is also known as corn, is a native plant to the North American continent. (All maize is also known as corn, and all maize is a native plant)
Nonrestrictive clauses are also used when the referent noun is unique: there’s no need to restrict it because it’s already fully identified:
The male part of the cob is the tassel, which releases pollen. (There’s only one part of the cob called the tassel and it always releases pollen.)
Henry A. Wallace, who was one of the most important corn researchers in the 1920s and 1930s, served as Vice President under Franklin D. Roosevelt. (There’s only one Henry A. Wallace and he was both a corn researcher and Vice President of the U.S.)
Since they expand rather than restrict meaning, nonrestrictive clauses can comment on the entire main clause:
Corn is an adaptable plant, which means it can grow in many different regions and climates. (Which = the fact that corn is an adaptable plant)
Form
There are two important differences in the form of nonrestrictive relative clauses compared to restrictive relative clauses:
The relative pronoun that cannot be used in a nonrestrictive relative clause.
There is always a comma before the relative clause to show that it is nonrestrictive. If the clause is in the middle of a sentence, a second comma is needed (much like these open and closed parentheses).
As with embedded (restrictive) relative clauses, the options for the relative pronoun in a nonrestrictive relative clause depend on whether the referent (the thing the pronoun refers to) is human or non-human:
|
Type of clause |
Human referent |
Non-human referent |
|
Subject relative clause (nonrestrictive) |
who |
which |
|
Object relative clause (nonrestrictive) |
who, whom |
which |
|
Object-of-a-preposition relative clause (nonrestrictive) |
preposition at the end: who, whom preposition at the start: whom (to whom, for whom) |
preposition at the end: which preposition at the start: which (to which, for which) |
|
Possessive relative clause (nonrestrictive) |
whose |
whose |
Once more, we can break up a sentence with a nonrestrictive relative clause into two sentences to see the function of the relative pronoun:
A good way to understand a sentence with a relative clause is to break it into two sentences. You should see that one participant (one noun group or pronoun) is repeated in both sentences. This is the referent noun that will be replaced with a relative pronoun. For example:
|
Type of clause |
Without a relative clause |
With a relative clause |
|
Subject |
Wallace was born in Iowa. Wallace started farming as a child. |
Wallace, who was born in Iowa, started farming as a child. |
|
Object |
Wallace met George Washington Carver. His father taught Carver at Iowa State College. |
Wallace met George Washington Carver, who(m) his father taught at Iowa State College. |
|
Object-of-a- |
Wallace created the Copper Cross hybrid. He won the Iowa Corn Yield Contest with the Copper Cross hybrid. |
Wallace created the Copper Cross hybrid, with which he won the Iowa Corn Yield Contest. Wallace created the Copper Cross hybrid, which he won the Iowa Corn Yield Contest with. |
|
Possessive |
Wallace was Secretary of Agriculture under Roosevelt. Roosevelt’s New Deal plan helped Americans through the Depression. |
Wallace was Secretary of Agriculture under Roosevelt, whose New Deal plan helped Americans through the Depression. |
Note the use of commas in all the nonrestrictive relative clauses! Replacing any of the pronouns in these examples with that would be considered ungrammatical in edited standard English.
Careful!
Some style guides argue that prepositions cannot be used at the end of a sentence. This is based not on actual usage, but the opinions of 19th-century writers, who you should pay no attention to. Clearly the previous sentence is perfectly grammatical and acceptable! You can of course move the preposition (to) up to the beginning of the relative in clause, but then you must you use whom not who (to whom you should not pay attention).
Sometimes, the distinction between a restrictive and nonrestrictive clause really is not clear, and proficient users of English will disagree over the need for the comma. You can make your best guess by finding the referent of the relative pronoun and asking whether it is fully identified: does the relative clause talk about all of the referent noun (use a comma), or restrict the meaning or one or some of the noun (don’t use a comma). For example:
About 95% of corn fields in the US are planed with hybrid corn, which has increased productivity. (nonrestrictive)
About 95% of corn fields in the US are planed with hybrid corn which has increased productivity. (restrictive))
The first sentence is probably better because we can assume that the use of all hybrid corn has increased productivity. The second sentence seems to suggest that some hybrid corn has increased productivity, but not all of it, and so should only be used if that is really the intention.
Function
Like restrictive relative clauses, nonrestrictive clauses add detail and information, expanding noun groups and sentences to make them more specific, interesting, and effective. Nonrestrictive relative clauses also improve cohesion (if you look at the examples in the table above, you should find the sentences without relative clauses feel choppy and repetitive). A further function is to show cause/effect, outcomes, results, and interpretations, and for this reason, they are quite common in texts that Explain.
