The period is perhaps the easiest punctuation mark to master. It ends a sentence. Difficulty generally arises only when the period is used with other punctuation marks.
This entry consists of the following sections:
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.”.
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.
If a sentence ends with a parenthetical that is only part of a larger sentence, the period is placed outside the closing parenthesis.
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.
Their house was the largest one on the block. (It also happened to be the ugliest.)
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.
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?”.
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.
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, a single space is generally preferred.
A series of three periods forms an ellipsis, which is explained here.