What is a comma?
A comma is a punctuation mark used to separate the elements of a sentence into sections, improving the clarity of the sentence.
How commas are used
With coordinating conjunctions to connect independent clauses
Coordinating conjunctions consist of and, but, or, for, nor, so, or yet.
Examples:
(Incorrect) The student did not come to class so he failed.
(Correct) The student did not come to class, so he failed.
After introductory elements, phrases, or words
Introductory clauses commonly begin with after, although, as, because, if, since, when, and while. Introductory phrases normally give the reader context for the sentence, helping the reader understand when or why the subsequent action took place. Some common introductory words include well, yes, and however.
Examples:
(Incorrect) Because the student was ill she made a doctor’s appointment.
(Correct) Because the student was ill, she made a doctor’s appointment.
In the middle of a sentence to set off nonessential clauses, phrases, and words
Place one comma before the nonessential information and one at the end. Information is nonessential if it can be removed from the sentence and the sentence still makes sense.
Examples:
(Incorrect) The topic of this class is really interesting! The professor on the other hand is rather boring.
(Correct) The topic of this class is really interesting! The professor, on the other hand, is rather boring.
To separate elements in a series of three or more
Examples:
(Incorrect) I have a paper a project and a presentation due by Friday.
(Correct) I have a paper, a project, and a presentation due by Friday.
To separate two or more coordinate adjectives describing the same noun
Coordinate adjectives have equal status within the sentence; for example, if the adjectives can be easily reversed and retain their meaning.
Examples:
(Incorrect) My favorite food is pepperoni, deep-dish pizza. (non-coordinate)
(Correct) My favorite food is a slice of cheesy, greasy, deep-dish pepperoni pizza.
(Correct) My cousin is a difficult, cranky child. (coordinate)
Near the end of a sentence to indicate a pause or shift in tone
Examples:
(Incorrect) I said I wanted a pizza not a hotdog.
(Correct) I said I wanted a pizza, not a hotdog.
To set off all geographical names
Examples:
(Incorrect) Columbus Ohio is the state capital.
(Correct) Columbus, Ohio is the state capital.
To set off items in dates (aside from month and day)
Examples:
(Incorrect) Josh was born on April 4 1990.
(Correct) Josh was born on April 4, 1990.
To set off addresses (aside from street number and name)
Examples:
(Incorrect) My new address is 1829 Oak Drive Kent OH 44240.
(Correct) My new address is 1829 Oak Drive, Kent, OH 44240.
To set off titles in names
Examples:
(Incorrect) Marvin Truss Ph.D. will be my professor next semester.
(Correct) Marvin Truss, Ph.D., will be my professor next semester.
To indicate a shift from the author’s own thoughts or writing to a quotation
Examples:
(Incorrect) In response to my question, she replied “I’d love to come to your birthday dinner!”
(Correct) In response to my questions, she replied, “I’d love to come to your birthday dinner!”
To prevent confusion or misreading of a sentence
Examples:
(Incorrect) To Hannah Mother Teresa was a role model.
(Correct) To Hannah, Mother Teresa was a role model.