The Four Basic Rules for Comma Usage

The Four Basic Rules for Comma Usage

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When positioned correctly, the comma prevents misreadings, assisting the reader in understanding the meaning of a sentence and how its parts fit together.  Review the following independent clauses as an example of a common misreading due to a missing comma:

Let’s eat Grandpa!
Let’s eat, Grandpa!

The first example (without the comma) is suggesting we eat Grandpa while the second (with the comma) invites Grandpa to eat.

Rule #1

A comma must precede a coordinating conjunction (FANBOYS) that joins two independent clauses. This comma marks where one independent clause ends and the next one begins.


    My brother studies at Harvard, but I will attend Cal State Long Beach.

Remember: Do not place a comma between coordinate word groups that are NOT independent clauses. In other words, do not separate two verb clauses that share the same subject. For example, a comma is unnecessary before “and” in the following sentence: I am hungry and want to eat a hamburger.

Rule #2

A comma should follow clauses that add information but are not necessary to the understanding of the meaning of the main sentence. These clauses include a) subordinate clauses, b) participle clauses that describe the subject immediately following them, c) transitional expressions, and d) introductory clauses that add information about when, where, why, or under what conditions the action of the sentence took place. This comma separates these types of clauses from the main sentence.


    a) While math problems have definite answers, there is no “right” way to write an essay.

    b) Using the chalkboard, the professor drew a brainstorming diagram.

    c) Alice is an English major; therefore, she will probably not make much money.

    d) During last summer’s camping trip, the park rangers led us on a hike.

Rule #3

A comma separates each item in a series that includes three or more words or phrases. Do not include a comma after the last item in the list, as this will separate the subject from its verb.


    Misused commas, dangling modifiers, and run-on sentences are some of the common grammatical issues in student writing.

Rule #4

A comma should be placed both before and after a parenthetical phrase that provides supplemental information or an afterthought.


    Writing, as far as I know, is a difficult and time-consuming process no matter how much experience you have.
    Both commas and semicolons, as we will examine later, function differently in a sentence.

Style Matters:

Now look at your own writing. Focus on one paragraph at a time and locate the commas in your sentences.  Based on what you now know, are they being used correctly?  Read your paragraph aloud.  Do you hear any natural pauses as you read that might need commas?  If so, make sure you refer to the rules again to ensure a comma is necessary.  This might be a good time to check for run-on sentences and comma splices.  Although placing commas where you naturally hear pauses in your writing is a good way to check for missing commas, simply placing commas where you might naturally take a breath is not.  Sometimes these pauses need to be punctuated with periods or semicolons, so continually refer back to the rules to make sure your comma placements are grammatically correct.

Copyright (C) 2010. All rights reserved.
This handout is part of a library of instructional materials used in California State University, Long Beach’s writing center, the Writer’s Resource Lab. Educators and students are welcome to distribute copies as long as they do so with attribution to all organizations and authors. Commercial distribution is prohibited.