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.

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.

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:

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.

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.

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

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

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

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

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.

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

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

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.

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

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

Separate two Independent clauses :
Use semicolon and comma

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.

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.

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.

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.

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

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

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.

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.

“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.

Direct Address

Use a comma to set off a direct address.

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.

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.