Commas tell the reader to pause, but the pause needs to be justified by a grammatical reason. Commas should never be placed in a sentence arbitrarily. However, if you don't know the rules regarding commas, you will either overuse them or neglect to place them where they should be used. Commas are used
- To join two clauses:
When Joe arrived at school, he realized that he'd forgotten to wear pants.
In this sentence, the comma joins a dependent and independent clause.
Joe arrived at school half-dressed, and he blushed from head to toe.
In this sentence, the comma and a conjunction join two independent clauses.
- After an introductory word:
However, Alex's dorm room was too small for his pet mongoose.
- Around nonessential clauses:
The philosophy professor, who drove a hot-pink Rolls-Royce, taught several classes at Skidmore.
A clause is not essential when it is not needed to define another word in the sentence. It's interesting to know that the professor drives a Rolls-Royce, but by placing commas around this information, the writer is telling us that this information is not needed to define which philosophy professor is the subject of the sentence. Suppose, though, there were two philosophy professors who might be the subject. Then the reader needs to know that the one who drove the Rolls taught several classes at Skidmore. In that case, you would omit the commas and write
The philosophy professor who drove a hot-pink Rolls-Royce taught several classes at Skidmore.
You as the writer, then, decide whether or not a clause is essential.
- In a series or a list:
Caroline's favorite foods include grapes, watermelons, tomatoes, and pineapples.
Note: Place a comma after the next-to-last item in the list, in this case "tomatoes."
- To separate two adjectives that describe a noun (only use commas here if you can switch the order of the words and not affect the meaning of the sentence):
Skidmore is a co-ed, liberal arts college or
Skidmore is a liberal arts, co-ed college.
Click here to read more about commas in the context of correct grammar.