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Use a comma

  • Before the conjunction in a list of three or more items. The comma that comes before the conjunction is known as the Oxford or serial comma.

Outlook includes Mail, Calendar, People, and Tasks.
Save your file to a hard drive, an external drive, or OneDrive.

Note: If a series contains more than three items or the items are long, consider a bulleted list to improve readability.

  • Following an introductory phrase.

With the Skype app, you can call any phone.

  • To separate independent clauses when they are joined by any of these seven coordinating conjunctions: and, but, for, or, nor, so, yet.

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Note: If the sentence is long or complex, consider rewriting as two sentences.

  • In a series of two or more adjectives that precede a noun, if the order of the adjectives can be reversed or if they can be separated by and without changing the meaning.

Adjust the innovative, built-in Kickstand and Type Cover.
PlayFab is a complete back-end platform.

Note: Consider rewriting for a friendlier, more conversational tone. For example, say, “Build mixed-reality apps that support collaboration across platforms,” not “Build collaborative, cross-platform mixed-reality apps.”

  • To surround the year when you use a complete date within a sentence.

See the product reviews in the February 4, 2015, issue of the New York Times.

Don’t use a comma

  • To join independent clauses when you don’t use a conjunction. Use a semicolon instead.

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  • Between verbs in a compound predicate (when two verbs apply to a single subject).

The program evaluates your computer system and then copies the essential files to the target location.

Note: Consider replacing a compound predicate with two sentences. Or add a subject for the second verb.

The program evaluates your computer system. Then it copies the essential files to the target location.
The program evaluates your computer system, and then it copies the essential files to the target location.

  • Between the month and the year when a specific date isn’t mentioned.

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