Comma Rules
- Use commas to separate independent clauses when they are joined by any of these seven coordinating conjunctions: and, but, for, or, nor, so, yet.
Example: I love vanilla ice cream, but my brother prefers chocolate. - Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.
Example: In the beginning, there was light. - Use a pair of commas in the middle of a sentence to set off clauses, phrases, and words that are not essential to the meaning of the sentence. Use one comma before to indicate the beginning of the pause and one at the end to indicate the end of the pause.
Example: Hilda, a very good cook, went to San Francisco. - Do not use commas to set off essential elements of the sentence, such as clauses beginning with that (relative clauses). That clauses after nouns are always essential. That clauses following a verb expressing mental action are always essential.
Example: It is critical that you not put a comma in this sentence. - Use commas to separate three or more words, phrases, or clauses written in a series.
Example: George traveled to Spain, France, and Germany. - Use commas to separate two or more coordinate adjectives that describe the same noun. Be sure never to add an extra comma between the final adjective and the noun itself or to use commas with noncoordinate adjectives.
Example: The big, hairy monster glared down at me. - Use commas to set off all geographical names, items in dates (except the month and day), addresses (except the street number and name), and titles in names.
Example: On October 3, 2015, Jeff Smith, marketing director at Intel, traveled to 14 Appian Way in Rome, Italy.
Courtesy of the Purdue OWL