Rhetoric and Composition/Commas

What Do Commas Do?

As you can see in the list below, commas serve several different purposes. For now, don’t worry about any unfamiliar terms; simply observe the main actions commas do: join, emphasize, contain, and separate.

They work with a coordinating conjunction to join two independent clauses within a sentence.
They emphasize introductory elements at the beginning of a sentence or clause.
They set off cumulative elements at the end of a sentence or clause.
They separate a dependent clause at the beginning of a sentence from the independent clause following it.
They work in pairs to contain restrictive modifiers within a sentence.
They work in pairs to contain parenthetical expressions within a sentence.
They separate two or more adjectives that independently describe the same noun.
They separate quotations from their attributions.
They separate items in a list.
They separate elements in dates, numbers, personal titles, and addresses.

While there are many different ways to use commas in writing, most comma usages fall into three situations. If you know the basic rule for these three cases, you should be set for comma usage.

Put a comma before a coordinating conjunction (for, and, nor, but, or, yet, so) that separates two independent clauses. Example: I wanted to drive to the mall, but my car wouldn't start.
Put a comma after introductory words, phrases, or clauses in a sentence. Example: Although it was a good offer, I felt that I needed to explore other options.
Use commas to set off elements that interrupt or add information in a sentence. Example: Tommy, my older brother, loved to punch me for telling his secrets.

Commas with two independent clauses

Used with coordinating conjunctions, commas allow writers to let readers know how their complete thoughts (as expressed in independent clauses) relate to one another. After all, our thoughts build on each other and interact with each other. Unless we’re merely listing things or free-associating, our thoughts rarely develop in separate “boxes,” but rather tend more to develop in relationship with one another.

When it’s time to put those thoughts into written text, commas used with coordinating conjunctions help us indicate the relationships among our thoughts. They help us avoid the choppy, flat style that comes from every thought becoming a separate sentence, walled off from other sentences with the finality of a period:

Example of periodic text: Building consensus ends with synthesis. It begins with analysis. Of course, the ultimate goal is finding commonality. The final product is a single course of action. However, a consensus derives validity only from agreement among the many. The first step in reaching consensus is to survey the different viewpoints involved.

If we add commas with coordinating conjunctions to this group of sentences, our reader will be able to see more clearly the relationships that exist among the complete thoughts:

The same text with commas and coordinating conjunctions: Building consensus ends with synthesis, but it begins with analysis. Of course, the ultimate goal is finding commonality, and the final product is a single course of action. However, a consensus derives validity only from agreement among the many, so the first step in reaching consensus is to survey the different viewpoints involved.

The formula

Use a comma to join two independent clauses (IC) with a coordinating conjunction (CC). Place the comma before the coordinating conjunction.

IC, CC IC.

Coordinating Conjunctions--For, And, Nor, But, Or, Yet and So--are often referred to by the mnemonic device used to memorize them: FANBOYS

Errors

A comma and a coordinating conjunction must appear together in order to have enough “strength” to join two independent clauses. These errors happen when one or the other is missing:

Two independent clauses joined by just a coordinating conjunction (no comma) or joined by nothing at all -- they just collide -- is called a run-on sentence or sometimes a fused sentence.

Examples of run-on or fused sentences:

Several environmental organizations recognized the treaty but few endorsed it.
Internet communities redefine the notion of space they take the concept beyond physical dimensions.

Two independent clauses joined by just a comma (no coordinating conjunction) is called a comma splice.

Example of a comma splice: Economists predicted lower personal debt loads resulting from tax cuts, this did not happen.

Commas with introductory words and phrases

Another relationship between thoughts we signal to our readers is that of introduction. We often want to give our readers some background before laying out our main thought, or we want to give some information first that limits or otherwise modifies the information in our main thought. Introductory elements can be one word or several so long as they do not contain a finite verb. An introductory element that includes a finite verb is likely to be a clause instead. Common ones are transition words and statements about time, place, manner, or condition.

Using a comma after an introductory element requires your reader to pause, and so should only be done with good reason.

Introductory Words

Introductory words are set off with a comma when the introductory word is a participle, modifies the entire clause following it, or when not including it might lead to misreading.

Examples of sentences with introductory words:

Humiliated, she fled from the diner. (participle)
Moreover, several groups actively opposed the treaty. (modifies entire clause)
Inside, traders shouted orders out. (Inside traders would be ambiguous)
Quickly tie your shoe (Quickly is a simple adverb, it only modifies tie.)

Introductory Phrases

Introductory phrases are not set off with a comma without good reason. Phrases that contain verbals, absolute phrases, long prepositional phrases, or compound prepositional phrase are set off. Short prepositional phrases are only set off for purposes of emphasis. Phrases that show inversion in sentence structure can also be followed by a comma.

Examples of sentences with introductory phrases:

Calling in sick for work, Beth hoped her boss would not suspect anything. (Phrase contains a verbal)
The stock market falling in Tokyo, Alex called his stock broker. (Absolute phrase)
Beneath the antique wooden fishing boat, barnacles had grown for years. (Long introductory phrase, but even here the comma is optional.)
Underneath the noses of her parents, Ruth had hoarded three kilograms of cocaine. (Compound prepositional phrase)
In a cold sweat, Henry read the letter addressed to his wife. (emphasis)
Drunk and angry, Joel burst into the room. (inverted structure)
After school I went to my uncle's house. (short prepositional phrase)
Before the parade I want to eat pizza. (short prepositional phrase)
By the earthen hearth my mother read to me from the book of Acts. (short prepositional phrase)

Commas with cumulative clauses

Another type of relationship between ideas that we signal to readers with a comma is that of accumulation. Occurring at the end of a sentence, cumulative clauses hook up to a main clause and add further information. Using cumulative clauses is a good way to avoid having to use two sentences when one will do.

Examples of sentences with cumulative clauses:

Nine senators changed their vote, passing the bill into law.
Three years of above-average rainfall raised the water table, turning formerly usable fields into wetlands.
Peers frequently reinforce the behavior, leading it to become an ingrained habit.

Commas and dependent clauses

Remember, when you read a dependent clause aloud, you or your listener has the feeling of “Well...? What’s next? Finish it up!” A dependent clause is a group of words that can’t stand on its own as a sentence because it does not express a complete thought. It leaves the listener (and reader) hanging.

Examples of dependent clauses:

although psychology and applied psychology are separate disciplines
when sanctions proved too difficult to enforce
because it was undated

Dependent clauses, as their name implies, "depend" on another clause to form a complete sentence. Dependent clauses must be paired with independent clauses.

The formula

When the dependent clause comes before the independent clause, use a comma after the dependent clause.

DC, IC.

When the dependent clause comes after the independent clause, just run right through without a comma.

IC DC.

Examples:

Although psychology and applied psychology are separate disciplines, they both are relatively recent additions to the university.
The U.N. approved military action when sanctions proved too difficult to enforce.
Archaeologists used contextual clues to date the manuscript because it was undated.

Errors

A disruptive comma is one used before a dependent clause that comes after the independent clause. Writers often make this mistake when the dependent clause begins with because.

Examples of disruptive commas:

The future of print newspapers appears uncertain, due to rising production costs and the increasing popularity of online news sources.
Some argue that print newspapers will never disappear, because of their many readers.

Commas and restrictive modifiers

You know what it's like to talk to someone and add explanations as you go, even right in the middle of your thoughts, to make sure your listener knows exactly which things you are talking about? When those explanantions show up in a written sentence, they are called modifiers. Modifiers can be nonrestrictive -- meaning that you can drop them out of a sentence and you won't change the meaning. Restrictive modifiers are ones whose meaning is essential to the overall meaning of the sentence; if you dropped a restrictive modifier, the meaning of the sentence would change.

Use a pair of commas to set off a nonrestrictive clause. Don't use commas around restrictive clauses.

Examples:

The committee, headed by Dr. Suarez, met weekly to develop a budget.
In this sentence, the phrase "headed by Dr. Suarez" is nonessential to the meaning of the main clause, which is that the committee met weekly. Presumably only one committee has been under discussion in previous sentences. However, if the discussion is of several committees, and the writer wants to point out that it was that one specific committee headed by Dr. Suarez that met weekly, the commas would be removed:
The committee headed by Dr. Suarez met weekly to develop a budget.
Employees who participate in the company's fitness classes pay a lower health insurance premium.
Here, no commas are used, since it is not just any and all employees who pay a lower health premium, but only those who participate in the company's fitness program. Notice the difference in the next sentence, in which the nonrestrictive modifier adds information but wouldn't change the meaning if it was taken out:
Employees, who have access to the company's gym and fitness classes, are encouraged to practice preventive healthcare.

Commas and parenthetical elements

Much like a nonrestrictive modifier, a parenthetical expression provides extra information or commentary in the middle of a sentence. A parenthetical element, however, often sounds more obviously "speech-like" and interjectory. Use a pair of commas to set off a parenthetical element.

Examples:

The candidate, much to the committee's surprise, voluntarily revealed her positions on several key controversies.
The question has, incidentally, since become moot.

Commas with multiple adjectives

When you use more than one descriptive word (adjective) to describe something (a noun), ask yourself whether the adjectives work independently to describe the noun, or if they build on each other and work together to describe the noun.

One way to tell is to reverse the order of the adjectives. If you can reverse the order and the meaning stays the same, the adjectives are working independently and should be separated by a comma. If you reverse the order and it doesn't make sense, the adjectives are working together and should not be separated by a comma.

Another test is to put "and" between the adjectives. If you can do that and retain the meaning, the adjectives are working independently and need a comma between them. If inserting "and" between the adjectives changes the meaning, the adjectives are working together and shouldn't be separated from each other by a comma.

If multiple adjectives before a noun work independently, use a comma between them. If they work together, don't.

Examples of adjectives working independently:

An open, exploratory, and inclusive spirit marked the meeting.
Test: An open and exploratory and inclusive spirit marked the meeting.
A direct, conversational tone made the instructions easy to understand.
Test: A conversational, direct tone made the instructions easy to understand.

Examples of adjectives working together:

Local health officials recently released guidelines for dealing with avian flu outbreaks.
Test: Health local officials recently released guidelines for dealing with flu avian outbreaks. Do not use commas.
An extruded plastic stem keeps the component stable.
Test: An extruded and plastic stem keeps the component stable. Do not use a comma.

Commas with quotations

Use a comma to set off the attribution -- the phrase that says who said or wrote a quotation -- from the quotation itself. Notice that the comma goes inside the quotation marks, even if the quotation is a complete sentence and would, if appearing on its own, take a period at the end.

Examples:

"The ballot is stronger than the bullet," writes Abraham Lincoln.
"Politics is the art of looking for trouble, finding it everywhere, diagnosing it incorrectly, and applying the wrong remedies," said Groucho Marx.

Use a pair of commas to set off the attribution when it appears in the middle of the quotation.

Example: "In a time of universal deceit," writes George Orwell, "telling the truth becomes a revolutionary act."

Do not replace a question mark or exclamation point in a quotation with a comma.

Example: "When will our consciences grow so tender that we will act to prevent human misery rather than avenge it?" writes Eleanor Roosevelt.

Do not use a comma to set off quotations that occupy a subordinate position in a sentence, often signaled by the words that, which, or because.

Examples:

Emphasizing the importance of staying in touch with the populace, James Madison wrote that "a popular government without popular information, or the means of acquiring it, is but a prologue to a farce or a tragedy, or both."
Participating in a democracy takes a strong stomach because "it requires a certain relish for confusion," writes Molly Ivins.

Commas in a list

Use commas between items in a list when there are three or more.

The final comma, the one before "and" or "or", is known as the Oxford comma, Harvard comma or serial comma. The Oxford comma should always be used where it is needed to avoid confusion (for example where one or more items in the list already include the word "and"). Otherwise it is optional. The Oxford comma is relatively uncommon in British English, except where used to avoid confusion. Not using the Oxford comma is relatively uncommon in American English, except in newspapers and magazines.

Examples:

Additional supplies required are a burner, beaker(,) and safety goggles.
The position requires expertise in building consensus, formulating policy(,) and developing long-range goals.
The English-speaking countries include Dominica, Trinidad and Tobago, and Jamaica.

Commas in dates, numbers, personal titles, and addresses

Use commas to separate the day of the week from the month and to set off a year from the rest of the sentence.

Examples:

"On Friday, February 13, 2015, we will be having our annual Valentine's Day dance.
On December 12, 1890, orders for the arrest of Sitting Bull were sent.
Graduation is set for May 20, 2006.

You do not need to use a comma when giving only the month and the year.

Example: The next presidential election will take place in November 2008.

Use commas to separate number into groups of three when they are more than four digits long. The comma is optional when the number is four digits long.

Examples:

2,400 (or 2400)
50,000
340,000

Do not use a comma in street addresses or page numbers.

Examples:

The table appears on page 1397.
The fire occurred at 5509 Avenida Valencia.

When following a name with a title, use a comma (if the title is at the end of the sentence) or two (if the title is in the middle of the sentence) to separate the title from the rest of the sentence.

Examples:

Paul Hjort, D.C., practices chiropractic medicine in central Minnesota.
Earnings far exceeded projections last quarter, according to Hitomi Masamura, Vice President for Operations.

Separate each element of an address is with commas. However, do not use a comma before a ZIP or other postal code.

Examples:

Bob Dole grew up in Hope, Kansas.
Write to the program advisor at 645 5th Street, Minerton, Indiana 55555.

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