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Grammar and Punctuation: Commas


Comma usage is, in some respects, a question of personal writing style. Some writers use commas liberally, while others prefer to use them sparingly. Most modern North American style guides now recommend using fewer commas rather than more. Therefore, when faced with the option of using a comma or not, you may find it wise to refrain (Peck, 2011).

This lesson will outline the key elements of how to use commas, and then allow you to test your knowledge using the educational tools provided.

Introductory Phrases and Clauses 

Use a comma when a sentence begins with an introductory infinitive clause, prepositional clause, participial clause, or subordinating clause. Note that there is always an independent clause (or complete sentence) following any type of introductory clause.

Use a comma after introductory infinitive clauses.

Example: To improve her English, she read for an hour before bed each night.

Use a comma after introductory prepositional clauses.

Example: Before he went to New York, he had spent a year in Australia.

Use a comma after introductory participle clauses.

Example: Having said this, he left the room.

Use a comma when a subordinating clause is used to begin a sentence. Common subordinating conjunctions include; although, unless, before, whenever, as long as.

Example: Whenever I'm on a road trip, I always keep a close eye on my fuel level.

Before Coordinating Conjunctions 

When using a coordinating conjunction (and, but, or, etc.) to link two independent clauses and make a compound sentence, place the comma before the conjunction.

Example: We spent all day at the river fishing, but we didn’t catch anything.

Example: I think I'll have the apple pie, or maybe I'll get the chocolate cake instead.

Surrounding non-essential words, phrases, or clauses

Use a pair of commas in the middle of a sentence to set off clauses, phrases, and words that are not essential to the meaning of the sentence. Use one comma before to indicate the beginning of the non-essential and one at the end to indicate the end of the non-essential.

Here are some clues to help you decide whether the sentence element is essential. If you answer yes to one or more of these questions, then the element in question is nonessential and should be set off with commas

  • If you leave out the clause, phrase, or word, does the sentence still make sense?

  • Does the clause, phrase, or word interrupt the flow of words in the original sentence?

  • If you move the element to a different position in the sentence, does the sentence still make sense?

Example of a non-essential word: I know Bill and Steve are out of work right now. I will, therefore, offer to pay for supper and drinks.

Example of a non-essential phrase: All the snowfall created great skiing conditions. The visibility, on the other hand, was terrible.

Example of a non-essential clause: Mr. Macleod, who teaches science to fourth graders, won a Nobel Peace Prize for physiology and medicine.

Commas and Lists

Most authorities on English, including the American Psychological Association, the Modern Language Association, and The Chicago Manual of Style recommend the use of the serial comma (see below). Use of the serial comma may add ambiguity or provide clarity depending on the situation. With that in mind, many sources also suggest neither systematically using nor systematically avoiding the use of the serial comma.

Example: My remaining assets are to be split among my wife, son, brother, and sister. 
 *Omitting the comma after brother would indicate that the brother and sister would have to split one-third of the estate

MLA Style Manual and Guide to Scholarly Publishing (2009) 3.2.2. Commas (p. 67)

American Psychological Association (2010) 4.03 Comma (p. 88)

The Chicago Manual of Style (2010) 6.19 Serial Commas (p. 312)

Practice using commas in a list of nouns, verbs, and adjectives.

Comma Splices

comma splice is an error caused by joining two independent clauses with only a comma instead of separating the clauses with a conjunction, a semicolon, or a period. A run-on sentence, which is also incorrect, is created by joining two independent clauses without any punctuation.

Time flies when we are having fun, we are always having fun. (Comma splice)
Time flies when we are having fun we are always having fun. (Run-on sentence)

Time flies when we are having fun; we are always having fun.
Time flies when we are having fun, and we are always having fun. (Comma is optional because both strong clauses are short.)
Time flies when we are having fun. We are always having fun.


This guide was created/compiled by Amanda Wills and last updated by Claire Pienaar in 2021. This work is licensed under a Creative Commons BY NC SA 4.0 International License.