Periods(and other terminal punctuation)
Use periods in the following instances:-- To indicate the end of a declarative sentence
The quartet sang for the Boy Scout banquet.
-- To indicate the end of an imperative sentence
Ship the order immediately.
-- To make a courteous request
Will you please reply promptly.
-- To punctuate abbreviations and initials
Dr. Val J. Hicks Mr. Smith 7:00 a.m.
-- To punctuate numerals or letters used in vertical lists.
-- When a sentence ends with an abbreviation, one period is sufficient for both the abbreviation and the end of the sentence.
Send the package to Smith and Webber, Inc.
Question marks and exclamation marksThese two terminal punctuation marks are used in obvious situations. Problems arise, however, when an editor uses them to produce emphasis. For this reason, use exclamation marks sparingly, and never in multiples. A strong statement will usually stand on its own without the addition of an exclamation mark. Some writers never use them.
Similarly with question marks, never use them in multiples or in combination with exclamation marks. Editors sometimes refer to these excessive punctuation marks as "screamers."