Comma, comma, comma, comma, comma chameleon...
The Three Most Common Comma Rules
While there are many specific uses for commas, nearly eighty-five percent of the commas used in written English are used in a mere three situations.
If you know the basic rule for these three cases, you can use commas in over four-fifths of the times you need to use commas.
1. Put a comma before a coordinating conjunction that separates two independent clauses.
The comma goes after the first clause and before the coordinating conjunction that separates the clauses.
Make sure they are independent clauses and not some other construction where commas are not required.
Correct: We washed the dog, and then we cleaned up the mess that he made.
Incorrect: We washed the dog, and then cleaned up his mess.
(There is only one subject. This is a single clause, not two independent clauses. The subject we has a compound verb.)
Correct: We washed the dog and then cleaned up his mess.
2. Put a comma after introductory words, phrases, or clauses in a sentence.
Commas are used to set off certain items that often begin a sentence and have no grammatical connection with the rest of the sentence. These items include certain common expressions, unemphatic interjections, and direct addresses.Common Expression: But of course, we have mustard in the car.
Unemphatic Interjection: Yes, we have no bananas.
Direct Address: Robert, please hand the man some mustard.
All three of these items are set off by commas no matter where they appear in the sentence. If they are not used at the beginning, the sentence often sounds more awkward.
Correct: Please, Robert, hand the gentleman some mustard.
Correct: We have mustard in our car, of course.
Introductory adverbs are normally set off by a comma unless they are followed directly by the word they modify.Correct: Clearly, one and one make two.
Incorrect: Clearly, mistaken was the witness.
(Clearly modifies mistaken which directly follows it because of a change in the word order.)
Correct: Clearly mistaken was the witness.
Do not use a comma between separate phrases unless they are in a series.
A comma may also set off a single prepositional phrase at the beginning to make the sentence clear. A comma is recommended after any introductory prepositional phrase of more than four words.Correct: Under the kitchen table the dog cowered.
(Single short, clear phrase. No comma needed.)
Correct: Under the spreading chestnut tree, the village smithy stands.
(Comma optional, but helpful due to length of phrase)
Correct: Under the pile of clothes, we found his wallet.
(Two prepositional phrases, not in a series)
Incorrect: On the sand, of the beach, by the inlet, we relaxed in the sun.
(Do not separate the phrases since they are not in a series.)
Correct: On the sand of the beach by the inlet, we relaxed in the sun.
Correct: Over hill, over dale, we hit the dusty trail.
(The two phrases are in series here. We could say "Over hill and over dale.")
Introductory Participial and Infinitive PhrasesCorrect: Looking for help, the man fell on his knees to beg.
Correct: To raise enough money in time, Mary had to issue stock in her business.
(The infinitive phrase is used as a modifier)
Incorrect: To ski, is exhilarating.
(The infinitive is used as a noun, not a modifier.)
Correct: To ski is exhilarating.
Place a comma after an introductory adverb clause.Correct: Before the curtain fell, the actors bowed.
Correct: If the next two nights are sellouts, the play will be extended.
Note that if a sentence ends with an adverb clause, no comma is used. The subordinating conjunction is enough of a separation.Incorrect: The play's run will be extended, if the next two nights are sellouts.
(No comma needed with adverb clause at end of sentence.)
Correct: The play's run will be extended if the next two nights are sellouts.
3. Use commas to set off elements that interrupt or add information in a sentence.
In addition to the items covered in Commas with Introductory Words, conjunctive adverbs are also set off by commas.
Conjunctive Adverbs are adverbs which join sentence parts. The following words are the most common conjunctive adverbs:
also besides furthermore however indeed instead moreover nevertheless otherwise therefore thus
Correct: John headed this way; however, he did not see me.
Correct: John headed this way; he did not see me, however.
Some adverbs which are used conjunctively may at times be used as a simple adverb. They are only set off by commas when used conjunctively or when some other comma rule applies.
Correct: John saw Kate; also, he saw Jean.
(Also here is joining the sentence parts.)
Incorrect: John saw Kate; he, also, saw Jean.
(Also here is simply modifying saw.)
Correct: John saw Kate; he also saw Jean.
Commas also set off contrasting expressions beginning with not.Correct: I wanted this one, not that one.
Correct: We went to New Hampshire, not New Jersey, for our vacation.
This is called a nonrestrictive modifier, i.e., it does not restrict the meaning of the noun or sentence.
Example: Any student not sitting down will get detention.
(This takes no comma because the phrase not sitting down is necessary to identify the noun. Remove it, and you get something very different: "Any student will get a detention.")
Example: Marcia Gomes, who was not sitting down, just got a detention.
(Here the person is named specifically. We know whom the sentence is about. The clause who was not sitting down does add information, but it is not necessary to identify the noun it modifies. Drop the clause and we still know who got the punishment: "Marcia Gomes just got a detention.")
Sometimes, the punctuation may depend on the situation. For example, if I have just one sister, or the reader already knows whom I am talking about, this sentence is correct:
My sister, Martha, is a nurse.
However, if I have more than one sister and it is not otherwise clear whom I am talking about, her name is essential to identify the sister.
My sister Martha is a nurse.
Or perhaps to make it clearer:
My sister Martha is a nurse; my sister Marianne is a teacher.