Use periods to end complete sentences that are statements, commands, requests, or mild exclamations. A few simple rules about using the period are demonstrated in the following examples.
He spends winters in Florida and summers on Cape Cod.
Please open your books to the third chapter.
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.
When she visits cities in the East, Tracey expects bad weather.
not When she visits cities in the East. Tracey expects bad weather.
If a question is asked as a courtesy, you can use either a question mark or a period. The period makes the question more routine and general; the question mark makes it more directed and personal.
Would you please take your seat before the bell.
Would you please take your seat before the bell?
Most common abbreviations end in a period: Mr., Dr., a. m., etc., Tues., Sept . Other abbreviations are written without a period. In general, you can omit periods for abbreviations written in capital letters ( FBI, CIA, IOU, CNN) if the abbreviation doesn't 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 ending in a lowercase letter should still be written with periods: yr., mo. Exceptions are mph, rpm, and metric measurement abbreviations such as ml, cm, and gm. Do not use periods with state abbreviations: AZ, CA, NY, WY, and so on. If you are uncertain about using periods with an abbreviation, check a dictionary or style guide.
Period with quotation marks
Always place the period inside quotation marks, whether or not it is part of the quotation.
Katie said, “I didn't take the money.”
Aunt Francine insisted on referring to her new puppy as “ my little tootsie wootsie.”
In the first sentence, the period is part of the quoted sentence. In the second sentence, the phrase my little tootsie wootsie does not end in a period, but the period for the entire sentence is still correctly placed inside the quotation marks.
Punctuation with abbreviations
If a sentence ends with an abbreviation, use only one period.
Twyla told her mother, “I won't be satisfied until I earn my Ph.D.”