Online Writing Lab

Comma Rules Explained

When it comes to punctuation, knowing when, and when not to, use commas in writing is the biggest problem most writers face.  Correct comma usage can be hard to learn, but once it is learned, writing becomes both easier and better. 

Many writers have been told to use a comma anytime they would pause while reading a piece of writing.  While following this suggestion will lead to correct comma usage in some situations, there are many other times when following this guideline will lead to unnecessary comma usage.   Instead of using this as a guideline, there are several specific rules that dictate when commas should be used.  Learning and practicing these rules will help any writer become better at using commas. 

Rule #1: Use a comma to separate independent clauses linked with coordinating conjunctions.

If you have what can be two separate sentences but want to make them one (creating a compound sentence), use a comma and a coordinating conjunction (for, and, nor, but, or, yet, so) to link them.  (More information on compound sentences) The comma should be placed in front of the coordinating conjunction. 

Examples:

  • My English Instructor was a good teacher, and he taught me a lot about the writing process.
  • We left the house later than we hoped, but we still made it to the concert on time.

Notice how the above sentences can be separated into two different sentences.  For instance, the first example could be written like this:

  • My English Instructor was a good teacher.  He taught me a lot about the writing process. 

This is also acceptable, but if we want to connect them into one compound sentence, both a comma and a coordinating conjunction are needed to make the sentence grammatically correct.

Note: Do not place a comma before a coordinating conjunction when it is used to link words or phrases. 

 Examples:

  • Correct: I like both English and math (no comma needed).  Incorrect: I like both English, and math (the comma isn’t needed).
  • Correct: He acted hungry but wasn’t (no comma needed). Incorrect: He acted hungry, but wasn’t (the comma isn’t needed). 

Rule #2: Use a comma at the end of an introductory element.

This rule can be confusing because introductory elements are often hard to identify.  Essentially, an introductory element begins a sentence by providing a transition from the last sentence or background information before the independent clause.  More on independent clauses)  Introductory elements come in the form of prepositional phrases, subordinate clauses, and transitional expressions.  Whenever one of these is used at the beginning of a sentence, a comma should be placed after it. 

Examples:

  • Prepositional phrase: In a hard fought contest, the home team prevailed after two overtimes. 
  • Subordinate clause: Because I did well on my final essay, I should be able to pass the class.
  • Transitional expression: For example, Aims offers a variety of services that can benefit students.

Note:  A comma is not always needed after short prepositional phrases or subordinate clauses, as long as leaving it out does not cause confusion for the reader.  However, using a comma after even a short prepositional phrase or subordinate clause is never wrong, so if in doubt, go ahead and use it.

Rule #3:  Use a comma to set off nonessential elements.

A nonessential element is a word, phrase, or clause that is not needed to complete a sentence.  In other words, it can be removed and the sentence still makes sense and is grammatically correct.  If removing the element changes the meaning of the sentence, it is essential.  Nonessential elements need to be offset with commas, both before and after. 

Examples of nonessential elements:

  • I went to the movies with my neighbors, Ron and Sally, and then we went to dinner.
  • The students in my morning class, ENG 121, like to participate in the discussions.
  • Her best friend, Heather, is planning a surprise party for her birthday. 

Rule #4:  Use a comma to separate items in a list or a series. 

A series or a list is defined as three or more.  Anytime there is a list of three or more items, use a comma to separate them.

Examples:

  • I went to the store and bought milk, eggs, bread, and fruit.
  • In my American literature class we read The Great Gatsby, All the King’s Men, As I Lay Dying, and the Grapes of Wrath.

Note:  There is often confusion about whether or not to place a comma in front of the word and in the last item of a list.  Generally, a comma should be placed in front of the and to separate the last item in the list from the one that proceeds it.  Without this comma, readers may think that the last two items are linked together in the list.  The basic rule is that when in doubt, the comma should be placed in front of the and. (This rule is often referred to as an “Oxford comma.”)

Rule #5:  Use a comma to separate multiple adjectives. 

If more than one adjective is used in a sentence, separate them with commas or by using and.  (This is also referred to as coordinate adjectives.)

Examples:

  • He was a tall, skinny man. 
  • Her shiny, red, expensive, sports car is envied by the neighbors.

Rule #6:  Use a comma to introduce a quotation.

Examples:

  • On the student’s paper, the instructor wrote, "Your thesis is well constructed but should be moved to the end of the introduction."
  • He said to me, "I appreciate your willingness to participate in the classroom discussions."

Rule #7: Use a comma with addresses, dates, and long numbers.

When using addresses in a sentence, whether specific or not, a comma should be placed between the street and city, between the city and the state, and at the end of the address.

Examples:

  • Aims Community College is located in Greeley, Colorado.
  • Use the address 5401 West  20th Street, Greeley, Colorado 80634, for any mail that needs to be sent to Aims Community College.

When using a specific date in a sentence, a comma should be placed between the day and the year and also after the year. 

Examples:

  • August 22, 2011, is the day I began my first semester of college.
  • The signing of the Declaration of Independence on July 4, 1776, led to the founding of the United States of America.

When using long dates in writing, place a comma every thousandths place, or to separate numbers into groups of three, beginning on the right.

Examples:

  • Denver is called the mile high city because it is roughly 5,280 feet above sea level. 
  • My new position will pay me a salary of $40,000 a year.

Using Commas Correctly