Here’s a simple comma function: a comma should come before a coordinating conjunction whenever the coordinating conjunction combines two complete sentences.
At this point, you might be saying, “Whoa there, partner! What are coordinating conjunctions again?”
Coordinating conjunctions, as their name implies, coordinate two sentences or items (coordinating), even as they conjoin them (conjunction). They are often called conjunctions, for short. There are only seven coordinating conjunctions. The mnemonic FANBOYS will help you remember them:
For
And
Nor
But
Or
Yet
So
Conjunctions can combine all kinds of things. They can combine words, phrases, or even full sentences. However, when we use a coordinating conjunction to combine two full sentences, we should put a comma before that conjunction to show that it is combining two large and complete parts. The comma also creates a pause and a drop in pitch just before the conjunction to emphasize the point that the conjunction is leading into another complete sentence. (Remember those two comma functions? If not, check ’em out! Here are links: commas create pauses and commas create drops in pitch.)
Here is an example of how a comma precedes a coordinating conjunction:
Last year, Bob taught three literature courses, and he served on the department’s hiring committee.
Notice that I am using and to combine two complete, stand-alone sentences:
1. Bob taught three literature courses
2. He served on the department’s hiring committee.
The comma before and acts as a separating agent, since it adds additional force to the conjunction. It says to the reader, “I am combining two big things here—sentences that could stand alone. Beware, reader: this single sentence involves not one, but two complete (but related) messages.” Considering the point that commas represent pauses in our writing, that comma also says, “Pause before this and to recognize that you are about to read another complete, stand-alone idea.” All of that—from a comma!
Here is a sentence that expresses the same essential idea as the sentence above. Notice, though, that it does not combine two full sentences; instead, it uses a compound verb:
Bob taught three literature courses and served on the department’s hiring committee.
There is no comma before and—and there doesn’t need to be. That is because and is not combining two complete sentences. It combines two verb phrases. Look:
Bob . . .
1. taught three literature courses (verb phrase—not a sentence)
and
2. served on the department’s hiring committee (verb phrase—not a sentence)
The presence of a comma (or lack thereof) in the sentences above acts as a visual cue so that readers know what is to come in the sentence. This is yet another function that commas serve in our writing.
Next Up: Can Comma Placement Be a Matter of Choice?
These comma articles have explored many applications of the comma, and many of these applications—like the pre-conjunction comma discussed in this article—are indeed matters of right or wrong, correct or incorrect, appropriate or inappropriate.
But are there cases in which comma placement is a matter of choice? For example, can we place a comma before a coordinating conjunction even if the “rule” discussed in this article says otherwise? Can we place some commas simply because we want to?
Sure we can! Read the next article to learn more. . . .
Christopher Altman is passionate about bringing the art of effective writing to everyday 
Americans. In addition to writing this blog, Mr. Altman produces and hosts The Writer’s Toolbox Podcast, and he is currently developing a number of book projects that examine the role of language in popular media and everyday life. His book, Myths We Learned in Grade School English, explores how adult writers can overcome the false writing rules learned in childhood. When he is not writing or teaching, Mr. Altman enjoys grilling out and savoring the mild summers of Central New York, where he is a professor of English at Onondaga Community College (Syracuse, NY).
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