This section explains the basic principles of punctuation. For more, please consult the Resources page.
A comma indicates a small break in sentence structure. Using commas effectively requires following a few rules, and some good judgement.
a. We use a comma to set off an element in a sentence; in that case, two commas are required:
Stopping for gas in Omaha, Nebraska, was necessary.
b. We use a comma to separate items in a series; a comma should precede the last element of a series, including before and or or:
I ate a burger, fries, a slice of pizza, and a bucket of wings.
I went to the supermarket, my friend stayed at home, and Robert sang a song.
You were sleeping, Michael was eating, and the neighbor was taking a shower and brushing her teeth.
Note: there’s no need for a comma between taking a shower and brushing her teeth, since this clause refers to actions performed by the same person, and the commas in this sentence separate people, not their actions.
c. etc., and so forth, and so on, and the like are preceded and followed by a comma (unless they are at the end of a sentence).
d. restrictive vs. nonrestrictive clauses/elements – a clause is restrictive if the sentence won’t make sense without it, and nonrestrictive if the added information can be omitted without changing the meaning of the sentence. As a rule, restrictive clauses are not preceded or followed by a comma; nonrestrictive clauses are preceded and followed by a comma (or other punctuation).
That is used for restrictive clauses, which for nonrestrictive ones. Both that and which may be substituted for who or whom; the function of the clause as restrictive or nonrestrictive determines the use of commas.
Examples of restrictive clauses (integral part of the sentence, no comma):
The food that I ate last night was poisoned.
The car that he was driving crashed into a wall.
The people who like to read are the ones who frequent this library.
Note: that can often be omitted, though this is purely a matter of style.
Examples of nonrestrictive clauses (can be omitted without changing the meaning of the sentence and hence require commas):
The book, which I enjoyed reading, was published last year.
This car, which is the fastest I have ever driven, has very comfortable seats.
I prefer the company of modest people, who do not try to appear more knowledgeable than they really are.
The lunch served at the dining hall yesterday, a musty pizza that has seen better days, was inedible.
The professor’s only book, “A Guide to Disassembling Engines,” never sold more than five hundred copies.
The professor’s book “The Meaning of Life” was a marvelous work of prose.
Note: constructed this way, the sentence implies the professor has written several books.
e. However, therefore, thus, indeed, such as, including, and so forth should be separated by commas according to their role in the sentence as restrictive or nonrestrictive expressions.
f. Small breaks, used to insert parenthetical expressions, are separated by commas. The rule here is a bit elusive: if only a short break is intended, use a comma. If a stronger break in your sentence is what you seek, use a dash or parenthesis:
None of the portraits, despite our efforts, remained untouched.
This student is, to say the least, unprepared for college.
The cheapest cakes in town, if not the tastiest ones, are sold in that bakery.
g. Independent clauses, joined to the sentence by and, but, or, yet, so, etc., are preceded by a comma. This rule does not apply to very short clauses where omitting the comma would not change the meaning of the sentence.
h. When a sentence is made up of two parts, each having its own verb and connected by and, there is no need for a comma:
He got into the car and drove home.
She ate the cake and then drank water.
i. Conditional sentences require a comma when the if clause precedes the then clause. If the order of the sentence is reversed, a comma isn’t necessary when the if clause is essential to the meaning of the sentence. See the Grammar section of this website for more on conditionals.
Note: the word then is almost always superfluous in a conditional sentence and should be avoided:
If the US invaded Iraq earlier,
then it would have prevented a lot of bloodshed.
A conditional sentence can be formed with other words but if, as the examples below show:
If you leave now, you will make it to the train on time.
Had she been here, she would have known everything.
When I get home, I will take a shower and eat dinner.
I’d like to see you in my office, if you don’t mind.
We will come with you if it does not rain.
They were amazed when they heard the news.
j. A comma is used to separate adjectives modifying the same noun when the comma replaces and. If one of the adjectives is essential to the meaning of the noun, however, a comma should not be used:
It is going to be a cold, lonely, miserable winter.
He has an older, wicked sister.
We have many intelligent colleagues.
I chose an affordable tasty chocolate birthday cake.
k. adverbial and participial introductory phrases are usually followed by a comma, but need not be if the meaning is clear without it:
Arriving late, the student had the temerity to ask the teacher for clarifications.
Having done my homework, I was ready to go to bed.
Driving right behind us was our crazy neighbor.
l. The introductory yes, no, and well, and the expressions that is (and or in the meaning of that is), namely, for example, and for instance should be followed by a comma.
m. Contrary to common practice, the structures not… but and not only… but do not require a comma:
They ate not only ice cream but also candy and waffles.
n. A comma may be used in dates:
May 22, 1998
but dates are often written without it:
15 January 2003
o. A comma should be used to indicate missing words that are understood from context:
In Philadelphia there were three accidents last night; in Washington, two; in New York, twenty.
p. Before quotation marks, a comma is often used to introduce the quotation. But a comma is not needed when that or whether immediately precede the quote:
She said, “he would like a piece of that cake now.”
She said that “he would like a piece of that cake now.”
A semicolon is used between two independent clauses to indicate some relation or connection between them that a period would not. A few rules to remember:
a. Each of the two clauses separated by a semicolon should include a verb (or more than one). Long or complex series are the exception (see c below).
b. A semicolon should precede the adverbs however, besides, therefore,accordingly, indeed, and the expressions that is, for example, and namely when they introduce an independent clause:
The state looked after the interests of its subjects; however, it failed to provide shelter and clothing when an earthquake hit.
I think I will leave school early today; that is, I will intentionally miss the Professor’s lecture.
c. In long or complex series, where some of the elements include internal punctuation, a semicolon should be used to separate the items:
Her friend came home and told everyone that she was robbed outside the supermarket; that her car, which she had bought a week ago, did not start; and that even though she seemed frightened, everyone ignored her.
The list of public institutions in Florence included: six hospitals, two of which were run by the Church; soup kitchens; three courthouses, of which one was permanently closed; and a granary.
3. Quotation marks:
We use quotation marks to:
1. quote another source or a person’s words verbatim.
2. clarify that certain words have a meaning beyond their literal one, or to indicate terms.
Note: it is usually better to italicize specialized terms than to place them within quotation marks.
When using quotation marks with other punctuation, the following rules should be applied:
a. Use single quotation marks to a indicate a quote within a quote:
And then she said, “do you really think that if the teacher tells you ‘please be quiet,’ you should continue your conversation?”
b. Commas and periods that come at the end of a quoted clause are placed before the closing quotation marks:
The historian argued that “the poor in Marseille were exposed to diseases and cold weather,” as well as inadequate housing condition.
In his book, the author argued that “Shakespeare did not write more than six plays.”
c. Semicolons are placed after the closing quotation marks:
The daughter said, “I want to go home, dad”; therefore, the father drove her back.
d. Exclamation points or question marks come either within the quotation marks (if they are part of the original text quoted) or right after them (if they are not part of the original text):
The woman shouted: “he stole my purse!,” but no one wanted to help her.
Did you believe him when he said: “I will never lie to you”?
e. Footnote numbers come after the closing quotation marks:
Marcus has argued that “in the course of a century or so Aleppo’s original role in East-West trade had thus been radically altered.”⁴
Coming soon: dashes, colons, periods, and parentheses.