Periods and Commas

Periods
Punctuation

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Periods end sentences. That's all they do.

Commas

Commas serve to set off separate parts of a sentence. The articles on phrases, clauses, and sentence types include several rules about comma use. For example, an introductory phrase is usually followed by a comma.

One of the most common punctuation errors is called a comma splice. It occurs when a writer tries to combine two independent clauses with only a comma. You can correct this error by replacing the comma with a period or a semicolon, or by adding a conjunction (such as "and" or "but") before the comma.