Between two complete sentences. Each sentence on either side of the semicolon must be complete with its own subject and verb. Keep in mind that a semicolon functions the same way as a period and a capital letter.
Please open the window; it is hot in here.
Between two complete sentences connected by words like however, therefore, consequently, nevertheless, and then.
We created study guides to help students prepare for the tests; however, students’ scores are not reflecting improvement.
Between items of a series that have internal commas.
The officers of the club are Ryan Marcilliat, president; Jennifer Blalock, secretary; and Chris Yow, treasurer.
We are thinking about moving to Omaha, Nebraska; Gainesville, Florida; or Memphis, Tennessee.
Between a complete sentence and a list or explanation, or preceding a long, formal quotation.
RULE OF THUMB 1: A complete sentence should precede a colon.
I hate this one course: English. (Yes, one item can constitute a list.)
Defending free speech, Voltaire said: “I may disagree with what you say, but I shall defend your right to say it.”
Three plays by William Shakespeare will be presented this summer: Hamlet, Macbeth, and Othello.
Ian was dissatisfied with the photos he had taken: all of them were out of focus.
The reasons he cited for his success are as follows: integrity, industry, and a pleasant disposition.
RULE OF THUMB 2: DO NOT place a colon after a verb (are, include, etc.), a preposition (such as, with, etc.), or namely or for example.
My favorite actresses are: Cecily Tyson, Betty Davis, and Lana Turner.
(Delete the colon.)
The box was filled with supplies such as: paper, pens, pencils, and notepads.
(Delete the colon.)
The box was filled with supplies, namely: paper, pens, pencils, and notepads.
(Change the colon to a comma.)