Commas are the most frequently used punctuation mark, and they are also the most common punctuation error.

Like other punctuation, the job of a comma is to help the reader understand how something would be said.  The word “comma” comes from the Greek word meaning “to cut off.”  The message commas send is this:  pause here, just for a second.

Try to read the following sentence:

I have three pigs four cats with six toes a gerbil named Hammy an old spotted cow who still gives milk and an Irish wolfhound named Vanessa.

You don’t know where to pause; you struggle to break the information into understandable chunks.  Now read this:

I have three pigs, four cats with six toes, a gerbil named Hammy, an old spotted cow who still gives milk, and an Irish wolfhound named Vanessa.

Commas help us translate words on the page into meaning.  Incorrect comma use makes the reader’s job much harder.  For example, look at these two sentences:

I love my parents, Beyonce and Barack Obama.

I love my parents, Beyonce, and Barack Obama.

The only difference is a comma.  If you put a comma after “Beyonce,” the sentence says you love four people. Without that comma, the sentence says that your parents are Beyonce and Barak Obama!

It’s the writer’s responsibility to use commas correctly, not the reader’s responsibility to figure out what the writer meant.

Basic Comma Rules

• Commas have two jobs: they either separate or they enclose.  Remember that, and you are halfway there.
• There are seven main rules for comma use.  Understanding these seven rules will eliminate nearly all of the comma errors in your writing.

Commas That Separate

Rule 1:  Use a comma to separate independent clauses joined by the conjunctions known as “fanboys.”  

For example:

• We brought chips to the party, and our neighbors were appreciative. (“We brought chips to the party” is an independent clause–it has a subject, a verb, and a complete thought.  “our neighbors were appreciative” is also an independent clause.  The two independent clauses are joined by the conjunction “and,” so we put a comma in front of “and.”)

• Tip

  This first comma rule is the most difficult. Master it and the others will seem easy!

  My geology textbook is expensive, so I’ll find a used copy.  (“My geology textbook is expensive” is an independent clause.  “I’ll find a used copy” is also an independent clause.  The two independent clauses are joined by “so.”  Put a comma in front of “so.”)

Caution: Be sure the conjunction connects two independent clauses, not just two words or two phrases or even two dependent clauses.  For example:

My dog curled up on the sofa and waited for dinner.  (“and” joins the verbs “curled” and “waited.”  The phrase “waited for dinner” is not an independent clause.  No comma before “and.”)

This rule clarifies a term you have probably heard: comma splice.  If we put a comma between independent clauses that are NOT joined by “fanboys,” that is a “comma splice.”  The comma is splicing, or cutting, the sentence in two parts.  A comma splice is a comma error.

Here is a comma splice:

Dogs are people’s best friends, people are a cat’s best friend.

To fix this error, add one of the “fanboys”:

Dogs are people’s best friends, but people are a cat’s best friend.

Understanding how a comma splice works has the added benefit of explaining what a run-on sentence is.  A run-on sentence is two or more independent clauses connected improperly.  For example:

I love to eat ice cream I would eat it every day if I could.

That run-on can be fixed by adding a comma and the conjunction “and” after “ice cream,” or by putting a period after “ice cream” and making two sentences.

Rule 2:  Use commas to separate items in a series, date, or address.  This comma allows the reader to pause after each item and identify which words are included in a group.

For example:

• We need to get flour, tomatoes, and cheese at the store.  (Separate items in a list with commas.  Note: Writers sometimes leave out the comma before the conjunction if the meaning is clear.  That is called an “Oxford comma” because it is the standard for writers at Oxford University.)
• Mr. Schaeffer could see a wild, dangerous, overgrown jungle in his neighbor’s yard.   (Use commas between a series of adjectives that modify a noun.)
• My grandfather was born on August 13, 1897, in Alameda County.  (In a date, put a comma between the day and the year.  If the sentence continues on, put a comma after the year.  When only the month and year are used, no comma is needed.  For example: He was born in August 1897.)
• My best friend’s address is 2600 Trillium Avenue, Mill Creek, Washington 97202, and she visits me often.  (Put a comma after the street and after the city, but not between the state and the zip code.  If you continue the sentence after the address, add a comma after the address.)

Rule 3:  Use a comma to separate an introductory word or phrase from the beginning of the main sentence.

For example:

• Finally, he received an Oscar for his work in film. (introductory word)
• During last season, our team won nearly every game. (introductory phrase)

Rule 4: Use a comma to separate a tag question, contrast, comment, or description from the end of the main sentence.  (A “tag” is an afterthought.)

For example:

• The age restriction goes into effect in March, doesn’t it?  (tag question)
• Students who earn high grades are those who study, not those who cheat.  (tag contrast)
• She said she would “consider my application,” whatever that means.  (tag comment)
• We spent a month in Italy, visiting family.  (tag description)

Most prepositional phrases and dependent clauses at the end of sentences are not tags and do not require commas.  For example:

The word “ruminate” means to think about something.  (“about something” is a prepositional phrase, not a tag.  No comma.)

Commas That Enclose

Rule 5: Commas are used to enclose (placed before and after) the name of a person being spoken to in a sentence.

For example:

Did you know, Sophia, that you left your book in class?

Rule 6: Commas are used to enclose transitions or expressions that interrupt the flow of the sentence.

For example:

I know, by the way, that my paper is late.

I will try, therefore, to be on time in the future.

If you are unsure whether an expression is interrupting, say it aloud.  You can hear the pause before and after “by the way” in the first sentence.

Rule 7: Use commas to enclose extra or unnecessary information in the middle of a sentence.

For example:

Max O’Keefe, who organized the event, will introduce the speakers.

But be sure the information is unnecessary:

The person who organized the event will introduce the speakers.  (We don’t know which person without the phrase “who organized the event.”  No commas.)