Use a comma to set apart an introductory word or phrase.
When a sentence doesn’t begin with the subject but has instead an introductory word or phrase, a comma must separate the introduction from the rest of the sentence.
- Despite his best efforts, the hero failed.
The comma goes between the introduction and the subject and must not separate the subject from the verb. Introductory elements often consist of prepositional phrases, subordinating conjunctions, participial phrases, or conjunctive adverbs. Phrases that begin with the following words often require a comma if they begin a sentence (this list is not exhaustive):
||in order to
The following words usually have a comma immediately after them when they begin a sentence. Many adverbs that end in –ly and transitions at the beginning of a sentence need to be followed by a comma, too.
||in other words
||in the end
||first (2nd, etc.)
||on the contrary
Finally, a comma is used to separate a participial phrase from an independent clause. A participle is a verb ending in –ing that acts as an adjective. Participles are often used with the above introductory words, but they can also stand alone.
- Running through the meadow, she ignored the demands of school.
- She was free as a bird, flying through the colorful fields.
When a verb immediately follows an introductory element, however, don’t use a comma. Such a case occurs if the introduction is serving as the subject of the sentence or if the sentence uses inverted word order. A comma is also sometimes optional after certain prepositional phrases. You can tell if you need a comma by whether you pause while reading the introductory element out loud. However, if you’re in doubt, use a comma.
- Running the race is the fulfillment of his dream. After the race came the victory lap.
- At night the stars come out.
Use a comma to join a dependent clause to an independent clause when the dependent clause is first.
A dependent clause is a clause with a subject and verb that can’t stand alone because of its first word (often one of the words from the first list above). If you were to say a dependent clause out loud, people would expect you to say more:
- Because we jumped in the pool.
When a dependent clause begins a sentence, a comma must follow it:
- Because we jumped in the pool, we were soaked.
Use a comma and a conjunction to join two independent clauses.
When you have two complete sentences—with two subjects and two verbs—you need more than a comma to separate them. You must join them either with a comma and a conjunction or with a semicolon, or you can make them two separate sentences. Joining two complete sentences with a comma is called a comma splice.
- I sat in my designated spot, my brother sat beside me. (Incorrect: Comma splice)
- I sat in my designated spot, and my brother sat beside me. (Correct: Comma and conjunction)
- I sat in my designated spot; my brother sat beside me. (Correct: Semicolon)
Use a comma to set apart nonrestrictive material.
Restrictive material narrows the meaning of the preceding noun:
- Children who play well with friends should be admired.
The above statement refers only to children who play well with friends. It is restrictive. Compare:
- Children, who play well with friends, should be admired.
This statement refers to all children as children who play well with friends, so it isn’t restrictive. The sentence would not change in meaning if you omitted the material enclosed by the commas. This is a useful test to see whether something is nonrestrictive: if a sentence would maintain its meaning without a certain phrase, then the phrase must be enclosed in commas. Students often use commas to set apart material that should be restrictive, which leads to confusion. Be aware of this. Finally, “that” is better for restrictive material and “which” is better for nonrestrictive material.
- The plant that thrives is fortunate. This project, which took her weeks to complete, is the best in the class.
Use a comma to set apart an appositive or an interrupting word or phrase.
An appositive is a word or phrase that repeats the preceding noun in different words, generally to amplify meaning. Any phrases that interrupt the sentence’s flow to provide information that the sentence could do without should have a comma on both sides.
- The cheetah, the fastest land animal, can reach speeds of seventy miles per hour.
- I mean, wow, who would have guessed?
Use a comma to separate items in a sequence.
When you list three or more items, it’s best to put a comma after every item except the last, including the one that precedes the “and” or the “or.” This way you can avoid possible confusion.
Use a comma to separate adjectives or adverbs that modify the same word.
Modifiers that reference the same word need commas separating them if there are two or more and if you could sensibly put the word “and” between them.
- It’s a tall, impressive building. The unusual, heavy box was strangely shaped.
Use a comma to introduce some quotations.
When there is a direct speaker, use a comma to introduce direct quotations. A colon is used for other kinds of quotations and citations. However, a quotation can also be its own sentence or be directly integrated into a sentence.
- The angry girl shouted at her younger brother, “Stay out of my room!”
Use a comma in dates, addresses, titles, academic degrees, and long numbers.
- After the day and, in a sentence, the year: On November 27, 1975, my big brother was born.
- After each part of an address: She’s at 698 Norfolk Street, Holliston, Massachusetts, where she works.
- To indicate a person’s title or degree: Julius Lombard, Ph.D., is my professor.
- After every three digits in a number: 5,980,672