Write Clearly: Punctuation

A comma is a punctuation mark (,) used to help readers understand how parts of a sentence are related to one another. If you're not sure about the use of a comma in a sentence, follow this guideline: “When in doubt leave the comma out."

Use a comma to separate items in a series or list of three or more words, phrases, or clauses. 

Examples: 

• He bought milk, (comma)eggs, (comma)and cheese.
• She woke up, (comma)ate breakfast, (comma)and brushed her teeth.

Note: The final comma before the “and” (known as the Oxford comma) is considered optional by some writers. We recommend including it to help keep list items separate.

Examples:

• Incorrect or confusing: I love my parents, (comma)Superman and Wonder Woman.
  • Note: In this sentence, readers may think the writer’s parents are Superman and Wonder Woman.
• Correct: I love my parents, (comma)Superman, (comma)and Wonder Woman.
  • Note: The comma after Superman helps readers understand there are three separate items [my parents, (comma)Superman, (comma)and Wonder Woman].

   

• Incorrect: Xiao decided to study sociology instead of philosophy, (comma)chemistry instead of biology and physics and history instead of geography. 
  • Note: It will be unclear to the reader which subjects are being substituted. 
• Correct: Xiao decided to study sociology instead of philosophy, (comma)chemistry instead of biology and physics, (comma)and history instead of geography. 
  • Note: The comma after physics helps readers understand that chemistry is being substituted for both biology and physics.

Use a comma before a coordinating conjunction when linking two complete sentences when both subjects are stated. Coordinating conjunctions can be remembered with the acronym FANBOYS (for, and, nor, but, or, yet, so).

Example:

• The professor moved to the front of the room, (comma)but the students didn’t say a word.
  • Note: "students" is the stated subject in the second clause. 

But if there is no subject in the second clause, do not use a comma before the joining word.

Example: 

• The professor moved to the front of the room but did not say a word.
  • Note: The "professor" is the subject of both verbs in this sentence.

The comma before a coordinating conjunction will help to clarify that you have a conjunction joining two complete sentences, as in the first example, rather than a conjunction joining two items in a series, as in this second example. Remember that no comma is required unless you have three items (see guideline #1). In the examples below, the comma is needed for that clarification. 

Examples: 

• Incorrect: The first vendor was selling ice cream with chocolate chips and worms were available from the second vendor. 
  • Note: Without the comma, the reader’s first impression is that the ice cream contains chocolate chips and worms.
• Correct: The first vendor was selling ice cream with chocolate chips, (comma)and worms were available from the second vendor.
  • Note: The comma here signals the reader that a second subject [worms] is being introduced.

Use a comma to separate introductory words, phrases, and clauses from main clauses. 

Examples: 

• Samantha, (comma)could you please help me?
• However, (comma)he didn't believe her. 
• Of all the reasons to stay home, (comma)that reason is the most ridiculous.
• Tired from working so hard, (comma)she decided to go home. 
• Although she was tired, (comma)she decided to keep working.

Use a comma, or a pair of commas, to separate from the rest of the sentence any word, phrase, or clause that is not essential to the sentence’s meaning. 

Examples: 

• My uncle Henri, (comma)who is almost seventy years old, (comma)has been sending me presents since I was born.
• Lighting a gas barbecue is easy, (comma)I think.
• The Great Dane, (comma)a large and gentle dog, (comma)is a great pet. 

Coordinate adjectives are those that modify the same noun and usually provide similar kinds of modifying information. Tip: Adjectives are coordinate when you could place “and” between them or change the order. Use a comma between coordinate adjectives. 

Example:

• Correct: Informative, (comma)imaginative writing can sell your ideas.

You could write Imaginative, (comma)informative writing can sell your ideas, or Informative and imaginative writing can sell your ideas. 

Note: Do not use a comma between the final adjective and the noun being modified.

Example:

• Incorrect: Informative, (comma)imaginative, (comma)writing can sell your ideas.
• Correct: Informative, (comma)imaginative writing can sell your ideas

Cumulative adjectives are those that provide different kinds of modifying information for a noun. In English, there is a typical order in which those adjectives are presented: quantity, opinion, size, age, shape or appearance, colour, origin, material, purpose. If a noun is used to modify another noun (for example, car parts), the modifying noun should come just before the noun. Tip: Cumulative adjectives cannot be joined with “and” or reordered. Commas are not needed between cumulative adjectives.

Examples: 

• Incorrect: One, (comma)gentle, (comma)white, (comma)old, (comma)large horse has been retired to the pasture.
  • In this sentence, the adjectives are providing different kinds of modifying information. They should be placed in the typical order before the noun, and no commas are needed. 
• Correct: One gentle large old white horse has been retired to the pasture.

• Incorrect: The gentle, (comma)loyal, (comma)racing horse has been retired to the pasture. 
  • "Gentle" and "loyal" are coordinate adjectives, so they do require a comma between them. However, "racing" is a cumulative adjective (it provides a different kind of modifying information than "gentle" and "loyal," and the order of "racing" in the sentence cannot be switched with the other adjectives). 
• Correct: The gentle, (comma)loyal racing horse has been retired to the pasture.  

• Incorrect: The teacher worked on a complex, (comma)computer program. 
  • In this sentence, “complex” modifies the phrase “computer program,” so you do not separate the two cumulative adjectives with a comma.
• Correct: The teacher worked on a complex computer program.

Do not use a single comma between a subject and its verb. 

Examples:

• Incorrect: Those two English courses, (comma)were so basic that I never really learned anything.
• Correct: Those two English courses were so basic that I never really learned anything.

• Incorrect: The group of nearly twenty members, (comma)provides opportunities often not feasible for the independent teacher.
• Correct: The group of nearly twenty members provides opportunities often not feasible for the independent teacher.

Note: You may use a pair of commas to enclose nonessential information between the subject and verb.

Example:

• Correct: Those two English courses, (comma)English 101 and 102, (comma)were so basic that I never really learned anything.

Do not use a comma between a verb and its object or between a preposition and its object.

Example of a verb and its object:

• Incorrect: It is even more interesting when you consider, (comma)that several species act the same way.
• Correct: It is even more interesting when you consider that several species act the same way.

Example of a preposition and its object: 

• Incorrect: I worked on the project with, (comma)two classmates. 
• Correct: I worked on the project with two classmates. 

Do not use a single comma after a coordinating conjunction.

Examples:

• Incorrect: I knew nothing about physics, (comma)yet, (comma)I passed the test easily.
• Correct: I knew nothing about physics, (comma)yet I passed the test easily. 

Note: you may use a pair of commas to enclose a nonessential expression after the conjunction. 

Example:

• Correct: I knew nothing about physics, (comma)yet, (comma)thank goodness, (comma)I passed the test easily.

Do not use a comma after “such as” or “like.”

Examples:

• Incorrect: We grow many types of fruit trees such as, (comma)cherry, (comma)apple, (comma)orange, (comma)and banana.
• Correct: We grow many types of fruit trees such as cherry, (comma)apple, (comma)orange, (comma)and banana.

Do not use a comma before a parenthesis. 

Examples:

• Incorrect: Mr. Sykes went to see her, (comma)(after school) but she was not there.
• Correct: Mr. Sykes went to see her (after school), (comma) but she was not there.