Restrictive vs. Non-restrictive Elements
A nonrestrictive adjective clause has a well-identified noun as its antecedent. Any of the information provided in the adjective clause does not serve to identify or specify the antecedent :
Larry Wilson, who had a bowling average of 215, was awarded the Fred "Twinkletoes" Flintstone Award in 1999.
This adjective clause is not needed to identify the antecedent because whe is already identified by name. Without the adjective clause, some details would be lacking, but the essential informantion would remain: Larry Wilson was awarded the Fred "Twinkletoes" Flintstone Award in 1999. Well-identified nouns include not only names of persons but also names of things, job titles, and any other phrases that plainly identify one of a kind:
The Crazy Horse Monument, which was dedicated in 1984, attracts visitors from all over the world.
We attended a reception for the Dean of the Graduate School, who will retire in June and attempt to break the world record for swallowing goldfish.
My oldest brother, who seldom had a girl friend for more than fifteen minutes, was named most-likely-to-survive-an-ugly-divorce by his senior class.
In all of these examples, the adjective clauses are nonrestrictive. They give information about the antecedents, but do not identify them. In each case the antecedents are already well-identified.
Sometimes the information included in the relative (adjective) clause can be additional information (also known as parenthetical information). If this is the case, then the relative clause should be separated out with commas.
A whatchamacallit, which is also known as a thingamabob, is just one of the many words that indicates the presence of plaques and tangles coming on.
A non-restrictive appositive is one where the noun that is replaced by the appositive is already well-identified.
V. S. Naipaul's first novel, The Mystic Masseur, told the story of the manipulation of the press by a clever Hindu.
In this case V.S. Naipaul can only have one first novel; therefore, this item is already well-identified or well-specified.
EXCEPTION: When the antecedent is the proper name of a group, the clause may restrict its meaning to certain members of the group and would therefore require no commas:
Most Americans who speak Arabic live in the city of Dearborn, Michigan.
Note: If you remove a nonrestrictive element from a sentence, the meaning does not change dramatically. Some meaning is lost, to be sure, but the defining characteristics of the person or thing described remain the same as before.
Alternative Ways to set off non-restrictive elements:You can also set off non-restrictive elements with em-dashes (using dashes is thought to place emphasis on the item set off by em-dashes)
The woman's favorite shirt—the one depicting Minnie Mouse being squeezed by a Burmese python—was disallowed by the school administration.
And you can also set them off with parentheses (in this case the parentheses diminishes the importance of the item inside . . . it trreats it as an aside)
Bozo the Clown (whom I have always considered to be harmless) turned out to be the culprit in the vending machine vandalism caper.
Do not use commas to set off a restrictive adjective clausečone that does identify the antecedent. The information may specify what kind of thing is being discussed, or the information may be specifying which members of a particular group. Often if the antecedent is a non-descript, generic noun, then the following relative clause will be restrictive.
Students who strive to set the curve by being determined to get the highest score will be tortured by the average students in the class.
This clause restricts the meaning of the antecedent, specifying which students are eligible for highest honors. Without the clause the sentence would say something quite different: Students will be tortured by the average students in the class. Since a restrictive clause is essential to the meaning of the antecedent and of the sentence as a whole, it must not be set off from the antecedent by commas.
Here is one more example:
The neighborhood vigilantes will have to remove the trailer that is parked on the street in front of our neighbor's house.
The appositive that specifies information about the preceding noun is considered to be a restrictive appositive. It is not set off by commas.
V. S. Naipaul's novel A House For Mr. Biswas describes his increasing pessimism about the impact of colonialism and its effect on the emerging nationalism in the developing world.
In this case V.S. Naipaul has many novels. The appositive "A House for Mr. Biswas" specifies which one of his novels that the author is talking about.
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Context and ambiguity in the restrictive vs. non-restrictive debate
Often it is difficult to tell whether a word group is restrictive or nonrestrictive without seeing it in context and considering the meaning.
Should you write:
"The dessert made with fresh strawberries was delicious" or "The dessert, made with fresh strawberries, was delicious"?
That depends. If the phrase made with fresh strawberries tells the readers which of several desserts you're referring to, you would omit the commas. If the phrase merely adds information about one dessert, you would use the commas.
The test for restrictive vs. non-restrictive clauses:
a) read the sentence without the adjective clause in it.
b) if, however, the meaning of the sentence is basically the same, then commas are needed [This is a nonrestrictive element].
c) if the meaning of the sentence is different [i. e. the omission of the phrase leaves out information which specifically identifies the antecedent] then no commas are needed [This is a restrictive element].
Exercises: Correct each sentence, if necessary
1. I had the pleasure of "hand-talking" to a lemur which had just returned from Madagascar where it had lived for ten years.
2. The American students knew by heart the exploits of Davy Crockett, a legendary American war hero, but they knew nothing about Wovoka or Meister Eckhart or any spiritual leader but their own.
3. Going on a treasure hunt in the desert with Juliette Binoche which has always been a fantasy of mine seems out of the question.
4. Sophocles' tragedy, Oedipus Rex, was given a splendid performance by the actor, Danny Kaye.
5. Martin and Eddie the little fox terrier who is his sidekick have warmed the hearts of audiences all over the world.
6. Farther along the highway, you will hit Reno which draws thousands of gamblers to its noisy casinos every year.
7. Tower Bridge whose colorful supports rise steeply above a winding river, that is always churning, is a rewarding place.
8. Here, in the spring, men and women who enjoy physical exertion and fresh air will be able to practice the long-held tradition of sheep herding.
9. In the late twentieth century, Charlton Heston who became president of the National Rifle Association led the fight for toy manufacturers to produce more realistic-looking toy guns.
10. Students who arrive late will be shot and left in the hall, where they will have their pockets gone through by small children.
Other sites with exercises:
Georgia State University
General comma usage rules
Comma Rules according to Public Works and Government Services in Canada