Commas are punctuation marks used to separate words, phrases, and clauses for clarity and pacing. Specific grammar rules in different style guides determine where to place commas, and some rules vary by discipline. While this handout lists general concepts, always work with your audience and assignment in mind.
An independent clause is a group of words that can form a complete sentence on its own. Two independent clauses connected by a coordinating conjunction (for, and, nor, but, or, yet, so) must have a comma before the conjunction.
A dependent clause is a group of words that depend on an independent clause to form a complete sentence. They often begin with a subordinating conjunction (although, after, because, if, until, when, as, while, etc.). A comma will generally come before the subject if other content comes before the subject of the sentence.
Commas are usually used after introductory 1) adverbs (often words that end in -ly); 2) participial phrases (phrases that begin with a verb ending in -ing or -ed); 3) infinitive phrases (when the word to precedes a verb); and 4) prepositional phrases (phrases that begin with a preposition and end with a noun).
Words, phrases, and clauses that can be removed from a sentence without changing its meaning are nonessential and are set off with commas. Appositives are words or phrases that rename a noun. Commas set off nonessential appositives, which provide nonessential information about a noun.
In contrast, commas do not set off essential words and phrases, which provide necessary information.
Commas set off the name or names of individuals being addressed.
Commas separate words and phrases in a list or series of three or more items. Some style guides or disciplines omit the comma before the coordinating conjunction (also called the Oxford comma).
Commas separate coordinating adjectives, which are multiple words that modify the same noun. Their order can be reversed, and they can be separated by the word and instead of a comma.
Commas are used to offset quotations in a sentence.
For places, a comma should separate the name of a city from its state, province, or country. If a city is listed with its state, province, or country, include a comma after the name of the state, province, or country as well. No comma is needed for only one place element.
For dates, a comma is needed between the date (August 18) and year (1980) and between the day of the week (Monday) and its date. If a day of the month (13), month (April), and year (1998) are listed together, a comma is needed after the year as well. No comma is needed for only one or two date elements. These comma rules use U.S. style; date formatting varies by country.