The rules for comma use are complicated. The recommendations included here are not exhaustive; they do, however, cover key sources of comma confusion in business communications.
3.1.1 Items in a Series
Use commas to separate items in a series, as in the following examples:
I ordered a salad, macaroni, and a soda.
The home furnishings company recently hired forty more salespeople, added two new business lines, and expanded its call center facility in Roanoke.
You may have noticed that in each of these examples, a comma appears before the and in the series. Many people were taught in school that this final comma is unnecessary, and it is in fact acceptable to omit it if you prefer that style. Journalists, for example, typically leave it out in accordance with Associated Press guidelines, and in business writing it is also frequently absent.
However, because the omission of the final comma sometimes leads to confusion, the preference of many grammar experts seems to be to keep it.
Whatever choice you make, try to be consistent. One caveat, though: even if you generally prefer to omit the final comma, you should include it whenever there is any ambiguity about the boundaries between the items in a series. For instance, ambiguity may occur when there is an and within one of the items, as in the following sentence:
I ordered a salad, macaroni and cheese, and a soda.
The second item in the series — macaroni and cheese — contains an and. Therefore, a comma should appear before and a soda; otherwise, the reader will have to sort through the words macaroni and cheese and a soda to figure out which items go together.
3.1.2 Coordinate Clauses
A comma should generally appear before a coordinating conjunction that combines two independent clauses (known in this case as coordinate clauses). In the following examples, clauses are indicated with brackets.
[I went to Staples to buy paper], and [Joan began binding the report].
[Last year the community college began offering special night classes for employees of local businesses], but [so far few people have signed up].
Sometimes if the clauses are very short, you may omit the comma, as in:
[The door slammed] and [Rex jumped].
However, it would also be correct to insert a comma after slammed. The choice in this sentence is really a stylistic one.
3.1.3 Introductory Phrases
You will sometimes find a phrase — a group of related words — at the very beginning of a sentence. Generally a comma appears after an introductory phrase, especially if it consists of three or more words.
Before the start of the conference, anxious presenters fortified themselves with pastries and coffee.
Even very short introductory phrases sometimes read better when they are followed by a comma.
In 1898, Manhattan and the Bronx merged with Brooklyn, Queens, and Staten Island to form modern-day New York City.
Still, commas after introductory phrases are often more about stylistic preferences than about rigid, unbreakable punctuation rules.
3.1.4 Parenthetical Words and Phrases
Sometimes a parenthetical word or phrase (parenthetical meaning that it is not essential to a sentence’s main idea) will interrupt the flow of a sentence. When this happens, commas should precede and follow the interrupting word or expression, as in these examples:
Elaine did not, as a matter of fact, show up for the brainstorming session.
Bob is allergic to wheat and corn. He is not, however, allergic to rice or bran.
Another type of interrupting expression is an appositive: a word or group of words that essentially renames the noun or pronoun that precedes it. Many appositives are set off with commas, as in this sentence:
My colleague, an accomplished pianist, is going to play for us at our upcoming staff retreat.
In this case, an accomplished pianist is the appositive — an alternative way to refer to the colleague.
In some cases, however, an appositive does not take a comma. The following sentences both include appositives and are both correct, but they mean two different things:
My son, John, is smart.
My son John is smart.
In each of the two sentences above, the presence or absence of commas tells us how to interpret the sentence’s meaning. In the first case, the commas around John tell us that the writer has just one son, and his name happens to be John. With this type of appositive, called a nonrestrictive appositive, you can think of the commas as indicating the nonessential nature of the information. If you take out the word John, the sentence still works — because there is only one son and there can therefore be no doubt about whom the sentence describes.
In the second sentence, the lack of commas indicates that the writer has more than one son. Here, John is a restrictive appositive, because it restricts the discussion to one specific son out of multiple sons. Without the name, you wouldn’t know which of the sons the sentence referred to; the name is essential information. Do not use commas with restrictive appositives.
3.1.6 Relative Clauses
Relative clauses are a particular type of clause beginning with pronouns such as who, which, or that.
The manager rewarded the employees who contributed most to the company’s success.
We passed by the old warehouse, which had been abandoned ten years earlier.
Smith Collins is the firm that I read about in the newspaper today.
One of the three sentences above includes a comma before the relative clause; the other two do not. In order to know how to punctuate these clauses correctly, you must first understand the distinction between restrictive and nonrestrictive relative clauses.
Restrictive clauses are essential for grammatical and logical completeness. Nonrestrictive clauses, in contrast, can be omitted without any significant loss of meaning and without disruption to the grammatical structure of a sentence.
Of the three sentences in the preceding examples, the first and third contain restrictive clauses. In the first sentence, the clause who contributed most to the company’s success restricts the class of all employees to those who most helped to ensure that the company would succeed. If the clause were deleted, the sentence’s meaning would be destroyed. The same holds true for the third sentence. Do not place commas before restrictive relative clauses.
The second sentence, however, contains a nonrestrictive clause. The clause adds information about the noun or pronoun that precedes it — in this case, warehouse — but removing it would have no significant impact on either grammar or meaning. Use a comma before nonrestrictive relative clauses.
People often use that and which interchangeably. In particular, it is common to see restrictive relative clauses beginning with which, perhaps in part because some people believe which sounds more elegant than that. However, among grammarians there is considerable support for a that–which distinction restricting the use of that to restrictive relative clauses and which to nonrestrictive. Observing this distinction can help ensure that there is no misunderstanding about your intended meaning. (See Section 5.4 for additional discussion of the difference between that and which.)
If you distinguish carefully between that and which in your writing, your relative clauses should usually be punctuated as follows:
- that clause: no preceding comma
- which clause: preceding comma
- who clause: preceding comma if clause contains nonessential information; no preceding comma if clause contains essential information
3.1.7 Compound Adjectives
When nouns or pronouns are preceded by multiple adjectives, it can sometimes be difficult to determine whether a comma is needed to separate the adjectives from one another. Consider these examples:
a talented British attorney
a talented, well-known attorney
The first contains no comma between talented and the second adjective (British); the second does. How do you know whether to include a comma?
Here’s a trick: try reading the phrase with an and between the adjectives. If it doesn’t sound right — a talented and British attorney does not sound right — then no comma is necessary. If adding the and sounds reasonable — and a talented and well-known attorney does — then you should insert a comma.
If a quotation is a complete sentence, place a comma after introductory words such as she says or they write. For example:
Wattley writes, “The government has failed the people.”
If the quotation is not a complete sentence, punctuate your sentence the same way you would if there were no quotation marks; in most cases, no comma will be necessary.
Wattley writes that the government “has failed the people.”
Please see Section 3.6 for additional details regarding punctuation placement around quotation marks.
3.1.9 Dates and Places
If a full date (month, day, and year) appears in a sentence, punctuate it as follows:
She was born on March 4, 2002, in Georgia.
If only the month and year appear, no commas are necessary.
She was born in March 2002 in Georgia.
If a city and state appear mid-sentence, they should be punctuated as in this example:
She was born in Atlanta, Georgia, in March 2002.
If you take away either the state or the city, though, the commas disappear. For example:
She was born in Atlanta in March 2002.
3.1.10 Dependent Clauses
If a complex sentence (see Section 2.2.3) begins with a dependent clause, you should generally insert a comma between the dependent clause and a subsequent independent clause.
Because it was raining, he took an umbrella.
However, if this sentence were reversed so that the independent clause came first and the dependent second, then no comma would be necessary.
He took an umbrella because it was raining.
These comma principles are not universally applicable, as there are sometimes grammatical or stylistic reasons to add or delete a comma where these sentence models would suggest otherwise. For the most part, however, the general guidelines described in this section hold true.
3.1.11 Comma Splices
A comma splice is a sentence error occurring when a writer combines two independent clauses with a comma and nothing more. It should be avoided. The following sentence, for example, is a comma splice.
The economic situation was dire, we therefore had to shut two of our stores.
One easy way to fix this problem is to replace the comma with a semicolon, whose use is discussed in more detail in the next section.
The economic situation was dire; we therefore had to shut two of our stores.