The Essential Clause
Recognize an essential clause when you see one.
Read these examples:
The man who ordered another double anchovy pizza claims to have a pet dolphin in his backyard pool.
Which man among the billions of human males on the planet? The one who ordered the double anchovy pizza!
Freddie hopes to return to the city where he met a woman with haunting green eyes.
Which of the many cities on the planet? The one where Freddie met a memorable woman!
The student who needs an A on the final exam is copying statistics formulae on her bare ankle.
Which of the many students in the class? The one who needs an A on the test!
Note that the exact same clauses above—in sentences with minor alterations—can become nonessential. Read these versions:
Mr. Hall, who ordered another double anchovy pizza, claims to have a pet dolphin in his backyard pool.
Freddie hopes to return to Cairo, where he met a woman with haunting green eyes.
Veronica, who needs an A on the final exam, is copying statistics formulae on her bare ankle.
In place of ambiguous nouns like man, city, and student, we now have Mr. Hall, Cairo, and Veronica, specific proper nouns. The information in the relative clauses might be interesting, but it's not necessary, for we already know which man, which city, and which student. Because these clauses are now nonessential, they require commas to separate them from the rest of the sentence.
A proper noun won't always signal that the relative clause is nonessential. In a passage of more than one sentence, you will sometimes find such a well-defined common noun that the relative clause is a mere accessory. Read this example:
As we sped through the neighborhood, we spotted crows eating French fries tossed on the road. They did not fly to a tree as we expected. The birds, which never showed fear of the vehicle, watched as we swerved around them.
The relative clause which never showed fear of the vehicle is nonessential since we know which birds. Thus the clause requires commas.
Punctuate essential clauses correctly.
Since an essential clause provides necessary limits on the vague noun it describes, use no punctuation to connect it.
The car that Madeline purchased from a newspaper ad belches black smoke whenever she accelerates.
The rats are nesting in the closet where Grandma hides her money.
The waiter who served the salad did not notice the caterpillar nibbling a lettuce leaf.
When the clause becomes decorative rather than defining—or nonessential—you will then need to separate it with commas:
The ancient Buick, which Madeline purchased from a newspaper ad, belches black smoke whenever she accelerates.
The rats are nesting in the master bedroom closet, where Grandma hides her money.
Javier, who served the salad, did not notice the caterpillar nibbling a lettuce leaf.