Comma usage is governed by both convention and grammar. For more details about comma usage, see Harbrace College Handbook.
In a series consisting of three or more elements, separate each element with a comma.
Microsoft style
You need a hard disk, a VGA monitor, and a mouse.
Not Microsoft style
You need a hard disk, an EGA or VGA monitor and a mouse.
Use a comma following an introductory phrase.
Microsoft style
In Windows, you can run many programs.
Not Microsoft style
In Windows you can run many programs.
If you specify a full date in midsentence, use a comma on each side of the year.
Microsoft style
The February 4, 2003, issue of the New York Times reported that .…
Not Microsoft style
The February 4, 2003 issue of the New York Times reported that .…
Do not join independent clauses with a comma unless you include a conjunction. Online documentation often has space constraints, and it may be difficult to fit in the coordinate conjunction after the comma. In these instances, separate into two sentences or use a semicolon.
Microsoft style
Click Options, and then click Allow Fast Saves.
Click Options; then click Allow Fast Saves. (Only if space is limited.)
Not Microsoft style
Click Options, then click Allow Fast Saves.
Do not use a comma between the verbs in a compound predicate.
Microsoft style
The Setup program evaluates your computer system and then copies the essential files to your hard disk.
The Setup program evaluates your computer system, and then it copies the essential files to your hard disk.
Not Microsoft style
The Setup program evaluates your computer system, and then copies the essential files to your hard disk.
Do not use commas in month-year formats.
Microsoft style
Microsoft introduced Windows 7 in October 2009.
Not Microsoft style
Microsoft introduced Windows 7 in October, 2009.
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