Skip navigation

Comma

Most often commas are used to separate items in a series or to set off introductory phrases. This section outlines the most common comma uses. Consider the following sentence:

Incorrect: While you were watching your favorite show I managed to finish the first assignment.

Correcting the Problem

In the sentence above, the adverbial clause while you were watching your favorite show needs to be set off by a comma.

Correct: While you were watching your favorite show, I managed to finish the first assignment.

Rule to Remember

Use a comma to separate introductory words, phrases, or clauses.

Introductory words and clauses

A comma is used to separate introductory words (besides, however, furthermore, therefore) and introductory phrases and clauses.

Correct: I need to spend some time preparing for this test. Besides, it's such an important test.

A comma is used to separate introductory participial phrases, adverbial clauses, and prepositional clauses.

Correct: Waiting for the storm to stop, I was anxiously looking out the window.

Appositives and parenthetical expressions

Commas are also used to set off appositives and parenthetical expressions.

An appositive is a word that follows a noun and explains it.

Correct: Anderson Cooper, a reporter, gained his first experience and eventual popularity by covering war stories.

Rule to Remember

Parenthetical expressions explain or clarify the sentence. They need to be set off by commas.

Parenthetical expressions also explain or clarify the sentence. Some of the more common expressions are of course, I believe, after all, by the way, in my opinion, for example, to tell the truth.

In the following sentence, I believe is a parenthetical expression.

Correct: The Big Bang Theory, I believe, provides a very interesting explanation of the origin of the universe.

And in this sentence, I believe is not a parenthetical expression.

Correct: I believe that The Big Bang Theory provides a very interesting explanation of the origin of the universe.

Such words as however, moreover, nevertheless, otherwise, or therefore often appear parenthetically.

Correct: I realize, however, that this theory may be flawed.

Non-defining clauses provide additional information about the noun they modify. These clauses are typically separated by commas.

Correct: Anderson Cooper, who is Gloria Vanderbilt's son, is an excellent reporter.
Correct: Thinking back, I realize now what kind of mistake I made.

Rule to Remember

Use commas to set off non-defining relative clauses.

Use a comma before a coordinating conjunction.

Compound sentences

When joining two complete sentences, use a comma before a coordinating conjunction (and, or, for, nor, so, but, or yet).

Correct: Laura developed the film, and Jason printed the pictures.

Dates, addresses, geographical names

Correct: Our last class reunion was on August 5, 1999, in Chicago, Illinois.

When only month and date are given, no comma is necessary.

Correct: We are meeting on August 5.

When listing items (either words, phrases, or clauses) in a series, use commas to separate them. There are different views on whether a comma is needed before and. Some grammarians have stated that it is not needed, and some have said that adding a comma before and prevents confusion in a sentence. Whichever rule you decide to go by, make sure you are consistent throughout your writing whenever there are series of items joined by and.

Correct: Laura showed Jason how to cut the negatives, dust the lens and counters, and change the ink ribbons in the printers.

Rule to Remember

When listing items (either words, phrases, or clauses) in a series, use commas to separate them.

Adjectives

When two adjectives separately modify the same noun and can be inverted and connected with and, use a comma between them.

Correct: Ignorant, unruly customers always seem to be the first ones in line.

Take Quiz