Multiple punctuation
If a sentence otherwise ends with a question mark or exclamation point, the period is omitted.
Correct
I've never seen Who's Afraid of Virginia Woolf?
Incorrect
I've never seen Who's Afraid of Virginia Woolf?.
Correct
He used to work at Yahoo!
Incorrect
He used to work at Yahoo!.
If a sentence ends with an abbreviation, the period used for the abbreviation also serves as the period for the sentence. This is true even if the abbreviation is contained within a quotation.
Correct
He is a vice president at Apple Inc.
Incorrect
He is a vice president at Apple Inc..
Correct
Laura said, “We will continue this tomorrow at 8:00 a.m.”
Incorrect
Laura said, “We will continue this tomorrow at 8:00 a.m.”.
Direct and indirect questions
An indirect question ends with a period, not a question mark.
Direct question
What is she doing tonight?
Indirect question
I wonder what she’s doing tonight.
Direct question
The question is, Does anyone support this legislation?
Indirect question
The question was whether anyone supported the legislation.
Proper placement of the period with parentheses
If a sentence ends with a parenthetical that is only part of a larger sentence, the period is placed outside the closing parenthesis.
Example
Hotel rooms are likely to be in short supply throughout August (the peak travel period).
If the parenthetical is itself an entire sentence, the period is placed inside the closing parenthesis.
Example
Their house was the largest one on the block. (It also happened to be the ugliest.)
Proper placement of the period with quotation marks
If a sentence ends with quoted material, the period is placed inside the closing quotation mark, even if the period is not part of the original quotation.
Example
The president’s speech both began and ended with the word “freedom.”
Note, however, that if the quoted material itself ends with a question mark or exclamation point, the period is omitted.
Correct
Yesterday he asked, “Why is it so cold on Mars?”
Incorrect
Yesterday he asked, “Why is it so cold on Mars?”.
Abbreviations
In addition to ending a sentence, the period is used with certain abbreviations. The current style is to use periods with most lowercase and mixed-case abbreviations (examples: a.m., etc., vol., Inc., Jr., Mrs., Tex.) and to omit periods with most uppercase abbreviations (examples: FBI, IRS, ATM, NATO, NBC, TX).
Note, however, that many scientific and technical abbreviations are formed without periods, even when they are lowercase or mixed-case. Examples: kHz (kilohertz), rpm (revolutions per minute), kg (kilogram), Na (sodium), 1st (first).
And a few uppercase abbreviations, including academic degrees, retain periods. Examples: U.S. (United States), J.D. (Juris Doctor), D.D.S. (Doctor of Dental Surgery). Some authorities, including The Chicago Manual of Style, favor omitting the periods in the previous examples.
Most established abbreviations can be found in a good dictionary, which will inform you of the use or nonuse of periods. See also the entry on style. Whether you choose to use periods or not, consistency is vital. It is inexcusable to write, for example, J.D. in one place and MD in another.
One or two spaces at the end of a sentence?
In the era of typewriters, it was common practice to insert two spaces at the end of every sentence. As long as you are typing on a computer, the best practice is a single space.
Ellipses
A series of three periods forms an ellipsis, which is explained here.