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Using Commas
Brought to you by the Purdue University Online Writing Lab at http://owl.english.purdue.edu.
The comma is a valuable, useful punctuation device because it separates
the structural elements of sentences into manageable segments. The rules
provided here are those found in traditional handbooks; however, in certain
rhetorical contexts and for specific purposes, these rules may be broken.
1. Use commas to separate independent clauses when they are joined by
any of these seven coordinating conjunctions: and, but,
for, or, nor, so, yet.
The game was over, but the crowd
refused to leave.
The student explained her question, yet the
instructor still didn't seem to understand.
Yesterday was her brother's birthday, so
she took him out to dinner.
2. Use commas after introductory a) clauses, b) phrases,
or c) words that come before the main clause.
a. Common starter words for introductory clauses that should
be followed by a comma include after, although, as,
because, if, since, when, while. (For
more information, see our handout on introductory clauses at http://owl.english.purdue.edu/handouts/grammar/g_commaint.html.)
While I was eating,
the cat scratched at the door.
Because her alarm clock was broken,
she was late for class.
If you are ill,
you ought to see a doctor.
When the snow stops falling,
we'll shovel the driveway.
However, don't put a comma after the main clause
when a dependent (subordinate) clause follows it (except for cases of
extreme contrast).
1. She was late for class, because her alarm clock was broken.
(incorrect)
2. The cat scratched at the door, while I was eating. (incorrect)
3. She was still quite upset, although she had won the Oscar. (correct:
extreme contrast)
b. Common introductory phrases that should be followed by a comma include
participial and infinitive phrases, absolute phrases, nonessential appositive
phrases, and long prepositional phrases (over four words). (For more
information, see our handout on verbals at http://owl.english.purdue.edu/handouts/grammar/g_verbals.html
and our handout on appositives at http://owl.english.purdue.edu/handouts/grammar/g_appos.html.)
Having finished the test, he
left the room.
To get a seat, you'd better come early.
After the test but before lunch, I went
jogging.
The sun radiating intense heat, we sought
shelter in the cafe.
c. Common introductory words that should be followed by a comma include
yes, however, well.
Well, perhaps he meant no harm.
Yes, the package should arrive tomorrow
morning.
However, you may not be satisfied with
the results.
3. Use a pair of commas in the middle of a sentence to set off clauses,
phrases, and words that are not essential to the meaning of the sentence.
Use one comma before to indicate the beginning of the pause and one at
the end to indicate the end of the pause.
Here are some clues to help you decide whether the sentence element is
essential:
- If you leave out the clause, phrase, or word, does the sentence still
make sense?
- Does the clause, phrase, or word interrupt the flow of words in the
original sentence?
- If you move the element to a different position in the sentence, does
the sentence still make sense?
If you answer "yes" to one or more of these questions, then the element
in question is nonessential and should be set off with commas. Here are
some example sentences with nonessential elements:
Clause: That Tuesday, which
happens to be my birthday, is the only day when I am available
to meet.
Phrase: This restaurant has an exciting atmosphere. The food,
on the other hand, is rather bland.
Word: I appreciate your hard work. In this case,
however, you seem to have over-exerted yourself.
4. Do not use commas to set off essential elements of the sentence, such
as clauses beginning with that (relative clauses). That
clauses after nouns are always essential. That clauses following
a verb expressing mental action are always essential.
That clauses after nouns:
The book that I borrowed from you is excellent.
The apples that fell out of the basket are bruised.
That clauses following a verb expressing mental action:
She believes that she will be able to earn an A.
He is dreaming that he can fly.
I contend that it was wrong to mislead her.
They wished that warm weather would finally arrive.
Examples of other essential elements (no commas):
Students who cheat only harm themselves.
The baby wearing a yellow jumpsuit is my niece.
The candidate who had the least money lost the election.
Examples of nonessential elements (set off by commas):
Fred, who often cheats, is
just harming himself.
My niece, wearing a yellow jumpsuit,
is playing in the living room.
The Green party candidate, who had the least
money, lost the election.
Apples, which are my favorite fruit, are
the main ingredient in this recipe.
Professor Benson, grinning from ear to ear,
announced that the exam would be tomorrow.
Tom, the captain of the team, was
injured in the game.
It is up to you, Jane, to finish.
She was, however, too tired to make
the trip.
Two hundred dollars, I think, is sufficient.
5. Use commas to separate three or more words, phrases, or clauses written
in a series.
The Constitution establishes the legislative,
executive, and judicial branches of
government.
The candidate promised to lower taxes, protect
the environment, reduce crime,
and end unemployment.
The prosecutor argued that the defendant, who was at the scene of the
crime, who had a strong revenge motive,
and who had access to the murder weapon,
was guilty of homicide.
6. Use commas to separate two or more coordinate adjectives that describe
the same noun. Be sure never to add an extra comma between the final adjective
and the noun itself or to use commas with non-coordinate adjectives.
Coordinate adjectives are adjectives with equal ("co"-ordinate) status
in describing the noun; neither adjective is subordinate to the other.
You can decide if two adjectives in a row are coordinate by asking the
following questions:
- Does the sentence make sense if the adjectives are written in reverse
order?
- Does the sentence make sense if the adjectives are written with and
between them?
If you answer yes to these questions, then the adjectives are coordinate
and should be separated by a comma. Here are some examples of coordinate
and non-coordinate adjectives:
He was a difficult, stubborn child.
(coordinate)
They lived in a white frame house. (non-coordinate)
She often wore a gray wool shawl. (non-coordinate)
Your cousin has an easy, happy smile. (coordinate)
The 1) relentless, 2) powerful 3) summer
sun beat down on them. (1-2 are coordinate; 2-3 are non-coordinate.)
The 1) relentless, 2) powerful,
3) oppressive sun beat down on them. (Both 1-2 and 2-3 are coordinate.)
7. Use a comma near the end of a sentence to separate contrasted coordinate
elements or to indicate a distinct pause or shift.
He was merely ignorant, not stupid.
The chimpanzee seemed reflective, almost
human.
You're one of the senator's close friends,
aren't you?
The speaker seemed innocent, even
gullible.
8. Use commas to set off phrases at the end of the sentence that refer
back to the beginning or middle of the sentence. Such phrases are free
modifiers that can be placed anywhere in the sentence without causing
confusion. (If the placement of the modifier causes confusion, then it
is not "free" and must remain "bound" to the word it modifies.)
1. Nancy waved enthusiastically at the docking ship,
laughing joyously. (correct)
2a. Lisa waved at Nancy, laughing joyously. (incorrect: Who is
laughing, Lisa or Nancy?)
2b. Laughing joyously, Lisa waved
at Nancy. (correct)
2c. Lisa waved at Nancy, who was laughing
joyously. (correct)
9. Use commas to set off all geographical names, items in dates (except
the month and day), addresses (except the street number and name), and
titles in names.
Birmingham, Alabama, gets its name
from Birmingham, England.
July 22, 1959, was a momentous day in his
life.
Who lives at 1600 Pennsylvania Avenue, Washington,
DC?
Rachel B. Lake, MD,
will be the principal speaker.
(When you use just the month and the year, no comma is necessary after
the year: "The average temperatures for July 1998 are the highest on
record for that month.")
10. Use a comma to shift between the main discourse and a quotation.
John said without emotion, "I'll
see you tomorrow."
"I was able," she answered,
"to complete the assignment."
In 1848, Marx wrote,
"Workers of the world, unite!"
11. Use commas wherever necessary to prevent possible confusion or misreading.
To George, Harrison had been a
sort of idol.
Comma Abuse
Commas in the wrong places can break a sentence into illogical segments
or confuse readers with unnecessary and unexpected pauses.
12. Don't use a comma to separate the subject from the verb.
An eighteen-year old in California, is now considered an
adult. (incorrect)
The most important attribute of a ball player, is quick reflex actions.
(incorrect)
13. Don't put a comma between the two verbs or verb phrases in a compound
predicate.
We laid out our music and snacks, and began to study.
(incorrect)
I turned the corner, and ran smack into a patrol car. (incorrect)
14. Don't put a comma between the two nouns, noun phrases, or noun clauses
in a compound subject or compound object.
The music teacher from your high school, and the football coach
from mine are married. (incorrect: compound subject)
Jeff told me that the job was still available, and that the manager
wanted to interview me. (incorrect: compound object)
15. Don't put a comma after the main clause when a dependent (subordinate)
clause follows it (except for cases of extreme contrast).
1. She was late for class, because her alarm clock was broken. (incorrect)
2. The cat scratched at the door, while I was eating. (incorrect)
3. She was still quite upset, although she had won the Oscar. (correct:
extreme contrast)
There are five exercises (with answer keys) on using commas that you
can try, available at the following URLs:
exercise 1: http://owl.english.purdue.edu/handouts/grammar/g_commaEX1.html
exercise 2: http://owl.english.purdue.edu/handouts/grammar/g_commaEX2.html
exercise 3: http://owl.english.purdue.edu/handouts/grammar/g_commaEX3.html
exercise 4: http://owl.english.purdue.edu/handouts/grammar/g_commaEX4.html
exercise 5: http://owl.english.purdue.edu/handouts/grammar/g_commaEX5.html
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