Use periods to end complete sentences that are statements, commands and requests, or mild exclamations. There are a few simple rules in using the period as in the following examples.
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He spends winters in Florida and summers in Cape Cod.
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Please open your books to the third chapter.
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How odd it is to see Robert sitting in his father's place.
Don't use periods at the end of phrases or dependent clauses. If you do, you create sentence fragments.
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When she visits cities in the East, Mrs. Tuhy expects bad weather.
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not When she visits cities in the East. Mrs. Tuhy expects bad weather.
Courtesy questions
If a question is asked as a courtesy, you can use either a question mark or a period. Although the difference in tone is slight, the period makes the question more routine and general, the question mark more directed and personal.
Abbreviations
Most common abbreviations end in a period: Mr., Mrs., a.m., etc., Tues., Sept. But other abbreviations are written without a period, and modern practice is shifting more in that direction. In general, you can omit periods for abbreviations written in capital letters (FBI, CIA, IOU, NBC) if the abbreviation doesn't appear to spell out another word. For example, USA is acceptable, but M.A. should include periods, since it could be mistaken for a capitalization of the slang word ma. Most abbreviations that end in a lower-case letter should still be written with periods: i.e., Dr., yr., mo. Exceptions to this rule include mph, rpm, and metric measurement abbreviations such as ml, cm, gm. Do not use periods with abbreviations for states: AL, AZ CA, NY, WY, etc. If you are uncertain about using periods with a particular abbreviation, check a dictionary or style guide.
Periods with quotation marks
Always keep periods inside quotation marks, whether or not they are part of the quotation.
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Katie Jane said, “I didn't take the money.”
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Mr. Wolfe insisted on referring to his young girlfriend as his “little tootsie pop.”
In the first sentence, the period is part of the quoted sentence. In the second sentence the phrase “little tootsie pop” does not end in a period, but the period for the complete sentence still correctly appears within the quotation marks.
Punctuation with abbreviations
If a sentence ends with an abbreviation, use only one period.
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He introduced his friend as Harold Ruiz, M.D.
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Tula told her mother, “I won't be satisfied until I earn my Ph.D.”
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