The rules below are based on the Chicago Manual of Style (CMOS). Please note that there are slight variations and preferences when using other style guides.

For rules on commas with independent clauses joined by coordinating conjunctions, beginning sentences with an introductory phrase, items in a series, and appositives, see also the Do I Need That Comma Flow Chart.

The Oxford Comma

When three or more items are written in a series, they should be separated by a comma. The Oxford comma is the comma that appears before the conjunction and is often left off. (See Comma Flow Chart for more on items in a series.) However, the Chicago Manual of Style recommends using it to prevent ambiguity. While this comma is optional, the author should be consistent in its use throughout and always use the Oxford comma if misreading could occur.

• The caterer provided food, tableware, and napkins.
• The horse ran through the field, around the barn, and stopped next to the water trough.

Etc., Et Al, and Other Such Terms

Terms such as etc., et al, so forth, and and the like are treated as the final element in a series.  Therefore, they are preceded by a comma, and a comma follows these terms only if the surrounding text requires it.

A special note on etc.—CMOS prefers to limit etc. to parentheses and notes in formal writing.

• The caterer provided food, tableware, napkins, etc.
• The caterer provided everything for the meal (food, tableware, napkins, etc.).
• The dog enjoyed doing normal “dog” things like lying in the sun, barking at the mailman, chasing rabbits, and so forth.

Restrictive and Nonrestrictive Clauses and Phrases

Now we begin a trickier set of rules where a comma being needed depends on whether the clause or phrase is restrictive or nonrestrictive. 

This section can be applied to:

• Dependent clauses that follow the main, independent clause (see flow chart for dependent clauses that begin a sentence)
• Relative clauses (clauses with “that” or “which”)
• Appositives (renames a noun in the sentence)
• Descriptive phrases
• Participial phrases

If it is restrictive (essential to understanding the meaning of the sentence or identity of the word to which it refers), then no commas are used.

• The horse will load in the trailer if you cluck to him. 
  • No comma is used because “if you cluck to him” is required to understand that the horse may or may not load if you fail to cluck to him.
• The girl jumped when her brother snuck up behind her.
  • No comma is not used because “when her brother snuck up behind her” is essential to understand why and even in what way she jumped. 
• The paper that I just finished took me two days to write.
  • No comma is used because “that I just finished” is required to know which paper.
• My brother Chris lives in Colorado.
  • Essential to understanding which brother lives in Colorado.  
• The dog with the blue collar needs a bath.
  • “With a blue collar” is essential to identify which dog needs a bath.

If it is nonrestrictive (not essential to understanding the main clause), then commas are needed.  If removed from the sentence, some information would be lost, but the meaning of the sentence would not change.

• The horse loaded in the trailer, after snatching a bit of grass.
  • The comma is needed because “after snatching a bit of grass” is supplemental information and does not impact the reader’s ability to understand that the horse loaded in the trailer.  
• The girl jumped into the water, even though she was scared.
  • Not essential to understanding that the girl jumped into the water.
• The “Do I Need This Comma” flow chart, which is also on the website, is helpful to understanding several comma rules.
  • Commas are needed because “which is on the website” can be removed without obscuring the identity of the flow chart.
• My oldest brother, Robert, lives in Wyoming.
  • Which brother is adequately identified by “oldest brother”.
• My dog, the one wearing the blue collar, needs a bath.
  • Supplementary information does not require commas to understand the sentence.

Adverbial Phrases

An adverbial phrase is a group of words that functions as an adverb in a sentence—it modifies a verb, adjective, adverb, clause, or sentence as a whole.

At the beginning of a sentence:
Introductory adverbial phrases can usually be followed by a comma, but it is not necessary unless misreading is likely. Shorter phrases are less likely to use commas.

• In 2014 I moved to Ohio.
• After the third and final try, I was able to hit the basket.
• Before eating, Grandpa always said grace.

In the middle of a sentence:
Use a comma in most cases.

• Grandpa, before eating, always said grace.

At the end of a sentence:
Use a comma only when the phrase is nonrestrictive.

• Grandpa always said grace before eating.
• I moved back from Ohio two years later, in 2016.

Do not use commas when an adverbial clause introduces an inverted sentence.

• Beside the dinner table stood the patriarch of the family.

Adverbial or Participial Phrases Used With a Conjunction

When following a coordinating conjunction, using a comma depends on whether the conjunction joins two independent clauses.

Joins two independent clauses:
Comma is placed before the conjunction

• We spent all day in the car, but in spite of the sedentary nature of travel, we were still worn out by the time we arrived.

Joins a compound predicate (share the same subject):
Comma goes after the conjunction

We spent all day in the car and, in spite of the sedentary nature of travel, were worn out by the time we arrived.

Introductory Words

A comma should be used with introductory words like yes, no, and OK. An exception can be made for informal prose or dialogue.

• Yes, you can go to the game.
• No, you can’t go without permission.
• Well, I guess we can make an exception.

Use a comma after oh or ah unless it is part of a phrase (oh boy).

• Oh, that’s right!
• Ah, I see.

Coordinate Adjectives

When two or more adjectives could be joined by and or reversed without changing the meaning, they are usually joined by a comma.

Another helpful tool I found in several sources that is NOT a CMOS rule involves using categories of adjectives. If both adjectives belong to the same category, they need to be separated by a comma. The order below is the typical order starting with the category that is listed farthest from the noun. Remember this is not a rule but a guideline you can use to help determine coordinate adjectives.

1. Determiner: articles (a, an, the), possessives, demonstrative (this, that, these, those)
2. Number
3. Observation or opinion: cold, pretty, carefree, heavy
4. Size
5. Shape 
6. Age
7. Color
8. Origin
9. Material
10. Qualifier – typically a word that further defines the specific type of item – ink pen, walking stick, fishing hook

• The clear, blue sky brought hope to the settlers.
• The bright blue sky was a reminder that better days lay ahead.
• The antique European coffee table was the centerpiece of the room.
• The expensive, ostentatious table was the centerpiece of the room.

Parenthetical elements

These elements can be set off with commas, em dashes, or parentheses depending on how strong of a break you intend. Commas are appropriate for only a slight break into the sentence.

• She felt better, relatively speaking, after taking her medicine.
• She felt better after taking her medicine, relatively speaking.

Conjunctive Adverbs

Words such as however, therefore, indeed, etc., are often set off by commas. However, it really depends on how it’s used in the sentence and surrounding sentence structure.

Used as a parenthetical, not essential to sentence:
Enclose in commas

• They were, however, the only ones to finish the race.

Essential to meaning and/or the emphasis is on the adverb:
No comma

•  The course was indeed treacherous that day. 

Separate two Independent clauses :
Use semicolon and comma

• The course was treacherous; therefore, only a few were able to finish the race.

Dates

A comma after the year in month/day/year format is often a forgotten but required punctuation.  When only the month and year are included, the comma is not needed. By the same token, when the day of the week is included, the month and day are set off by commas.

• On July 4, 1776, the Continental Congress declared its independence from Britain. 
• I have Friday, July 3, off because the Fourth of July is on Saturday.
• July 1776 was the year America declared independence from Britain.
• Christmas 2018 was our first Christmas without Dad.

Addresses

A comma is needed after the state name when city and state are included. When giving a complete address, there is no comma between the state and zip code.

• The business address is 123 Main St., Somewhere, Alabama 12345.
• Somewhere, Alabama, is not far from the Gulf of Mexico.

Commas with Quotations

Other than text introduced with a conjunction, text that introduces a quote is followed by a comma. A comma is also placed inside the closing quotation mark when interrupted by narrative text.
Text introduced with a conjunction (that, whether, if, etc.) does not require a comma before the quotation.

• He said, “I wasn’t there when the package was delivered.”
• “I wasn’t there,” he claimed, “when the package was delivered.”
• “I wasn’t there when the package was delivered,” the man answered.
• Who said that “to err is human, to love divine”?

Commas with Questions

Sometimes a direct question is included in a sentence without the use of quotations. When this happens, use a comma and start the question with a capital letter. Basically, treat it as if it was in quotes.

• The boy wondered, What happens if I don’t pass the test?

If the question ends before the end of the sentence, do not use a comma.

• What happens if I don’t pass the test? the boy wondered.

Indirect questions do not need a question mark, are not set off with a comma, and are not capitalized.

• The boy wondered what would happen if he didn’t pass the test.

Jr., Sr. Inc., And Similar Extensions

Commas are not required with Jr. and Sr. unless it is an inverted name. However, if you choose to use the comma, a second comma is required afterward. The use of Roman numerals such as II and III never use a comma as part of the name except when the name is inverted.

• Jim Smith Jr. was the first to arrive.
• Jim Smith, Jr., was the first to arrive.
• Smith, Jim, Jr.
• Smith, Jim, III

Commas are not required for Inc., Ltd., etc when used as part of a company’s name. Some companies choose to use commas in company documentation. Consistency is key. As with the case of Jr. and Sr. above, when Inc. and the like are set off by commas, a following comma is normally required.

• My Company Inc. was established in 2010.
• My Company, Inc., was established in 2010.

“Too” and “either”

When used to mean “also” at the beginning or end of a sentence, no comma is needed. When used in the middle of a sentence, the comma is useful for clarity.

• He wanted to go to the store too.
• He, too, wanted to go to the store.

Direct Address

Use a comma to set off a direct address.

• Amanda, please feed the cat when you get home.
• Please, Amanda, feed the cat when you get home.

Using i.e. (that is) and e.g. (for example)

It is preferred to limit these terms to parentheses or notes and follow with a comma.

• Many of the coniferous trees in the local forest (i.e., evergreens and pines) were over a two hundred years old.

When using terms such as that is, namely, or for example, it is best to precede them with a semicolon or em dash and follow with a comma. You can also enclose the entire phrase with em dashes or parentheses.

• Many of the coniferous trees of the local forest—that is, evergreens and pines—were over two hundred years old.
• Many of the coniferous trees in the local forest; for example, evergreens and pines, were over two hundred years old.