The period (known as a full stop in British English) is probably the simplest of the punctuation marks to use. You use it like a knife to cut the sentences to the required length. Generally, you can break up the sentences using the full stop at the end of a logical and complete thought that looks and sounds right to you.
MARK THE END OF A SENTENCE WHICH IS NOT A QUESTION OR AN EXCLAMATION
- Rome is the capital of Italy.
- I was born in Australia and now live in Indonesia.
- The Dalai Lama is the spiritual leader of the Tibetan people.
INDICATE AN ABBREVIATION
Many abbreviations require a period. Dr, Mr, Mrs, and Ms do not take a period in British English, nor do most abbreviations taken from the first capital letters such as MA, Phd, or CIA. In American English, some of these do require periods or both usages are correct (with and without periods). If you require 100% accuracy in your punctuation, refer to a detailed style guide for the abbreviation usage rules in the variety of English you are using.
- I will arrive between 6 a.m. and 7 a.m.
- We are coming on Fri., Jan. 4.
Often you will see a sentence concluding with three dots. This indicates that only part of the sentence or text has been quoted or that it is being left up to the reader to complete the thought.
- The Lord's Prayer begins, "Our Father which art in Heaven..."
- He is always late, but you know how I feel about that...
PERIOD AFTER A SINGLE WORD
Sometimes a single word can form the sentence. In this case you place a fullstop after the word as you would in any other sentence. This is often the case when the subject is understood as in a greeting or a command.
PERIODS IN NUMBERS
Numbers use periods in English to separate the whole number from the decimal. A period used in a number is also called a "decimal point" and it is read "point" unless it refers to money.
- $10.43 = ten dollars and 43 cents
- 14.17 = fourteen point one seven
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.
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.)
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.
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.