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There is no space before a period and only one space after. In the past (the days of the old typewriters), two spaces were used after a period, but thanks to modern kerning, this is no longer necessary. With quotation marks, periods go inside the closing quotation mark.
1. To end a complete sentence
Example: The dog chased the squirrel.
2. Abbreviations (General rule: use periods with abbreviations that appear in lowercase, but don’t use periods with those in uppercase)
Example 1: Mr., Mrs., Dist. Atty., Dr.
Example 2: e.g., i.e., etc., a.ka., USA, NASA, CEO
3. To punctuate a fragment
Example: She stopped dead in her tracks. Frozen.
4. A rhetorical question (Style note: Depending on the feel and/or tone of the rhetorical question, either a period or a question mark can be appropriate.)
Example 1: What difference does it make.
Example 2: How many times do I have to tell you to put your dirty clothes in the hamper.