The Comma

When do I use a Comma?

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.

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):

according to	because	despite	in order to	since	when	with
after	due to	instead of	though	whenever	without
although	before	even though	like	toward	where
as	besides	except for	once	unless	wherever
at	between	if	rather than	until	while

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.

additionally	finally	instead	next	therefore
after all	first (2nd, etc.)	last	of course	undoubtedly
afterward	furthermore	likewise	on the contrary	thus
again	however	meanwhile	otherwise	still
also	in other words	namely	presently
anyway	in the end	naturally	regardless
consequently	indeed	nevertheless	similarly

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.

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.

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:

When a dependent clause begins a sentence, a comma must follow it:

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.

Use a comma to set apart nonrestrictive material.

Restrictive material narrows the meaning of the preceding noun:

The above statement refers only to children who play well with friends. It is restrictive. Compare:

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.

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.

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.

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.

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

Reference: Strunk, Wiliam Jr., and E. B. White. The Elements of Style. 4th ed., Allyn and Bacon, 2000.

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