1. A period ends a sentence that is not a question or an exclamation. The period at the end of this sentence is an example.
2. Periods follow abbreviations and contractions—although this is becoming less common, especially in the use of acronyms and initialisms. In British and Australian English, it is standard practice to use periods when abbreviating lowercase words (such as e.g., etc., a.k.a.) and no periods when the original words are capitalized (e.g., USA, CEO, MIT, NATO, UK). This appears to be becoming standard in American English as well. Some publications, particularly in the United States, still use periods in all abbreviations, but the overall trend is toward fewer periods. Americans still tend to use a period following a contraction, such as Mr. or Dr. or Hon., while Britons, Australians and New Zealanders are apt to write Mr, Dr and Hon—that is, without a period or full stop.
When there is a lowercase abbreviation at the end of a sentence, use only one period—for example:
In the real world, pesticides aren’t used alone but in combination with other pesticides, fungicides, herbicides, etc. The synergistic effects can be … [NRDC Switchboard]
Parentheses and periods
When a standalone sentence is enclosed in parentheses, the period goes inside—for example:
On MSNBC, Chris Matthews worried what would happen if al-Qaida started dispatching bombers trained in martial arts. (Maybe we’ll need to deploy Matt Damon’s stunt double after all.) [Salon]
When parenthetical content is enclosed in a separate sentence, even when it’s an independent clause, the period always goes outside—for example:
The device has a second microphone on the back that monitors inbound noise and automatically cancels it out (anyone who’s used Bose noise cancellation headphones on a long flight will appreciate this). [Tech Crunch]
Quotation marks and periods
In American English, periods go inside quotation marks, even when the quoted text has no period:
Portland’s County Cork Public House on Northeast Fremont Street is named for the Contae Chorcaà, one of the traditional counties of Ireland also known as “The Rebel County.” [Brew Public]
In British and Australian English, the period goes outside the quotation marks. For more details on this, see the rules for quotation marks.
Double space or single space after a period?
Most publications use one space, but this is a subject of endless debate among writers and usage authorities. If two spaces look better to you, then feel free to use the double space, but if you look closely at the books, newspapers, and magazines you have at hand, you’ll see that most, if not all, use single spaces.