The colon is a punctuation mark that, when put after a statement, signals to the reader that there is more information to follow. It relates in some way to the original statement and is used to emphasize a point, expand upon or explain an idea, or to introduce a quote or a list. The main thing to remember with a colon is that it must always come at end of an independent clause (also known as a complete sentence). In other words, if you substitute the colon with a period, the original sentence should still make sense.
The following are examples of using a colon correctly in a sentence:
Correct: There are three things on my grocery list: eggs, milk and bread.
Correct: It was Albert Einstein who said it best: “A person who never made a mistake never tried anything new.”
Correct: She was only worried about one thing: how to write an article on colons.
Here, the items in the list are separated by commas—it’s also acceptable for them to be bulleted, numbered or lettered. Deciding whether or not to capitalize after the colon is also a stylistic choice (unless you have a proper noun) and neither is right or wrong, as long as it’s consistent. Since the colon tends to grab attention, particularly when used in creative writing, you might consider using it sparingly, for the greatest impact.
It’s important to remember that a colon should never come directly after a preposition or a verb, so the following would be wrong:
Incorrect: The three things on my grocery list are: eggs, milk and bread.
If you replaced thea colon with a period in the above example, the result would be an incomplete sentence.
A colon may also be used for the following situations:
A salutation in a formal/business letter.
Correct: Dear Mr. Jones:
Correct: To Whom it May Concern:
To clarify or add to a title, such as a book or movie
Correct: Unbroken: A World War II Story of Survival, Resilience, and Redemption
Correct: Star Wars: The Clone Wars
To separate numbers, like hours, minutes and seconds when writing the time or to show ratios
Correct: 4:30 PM
In lieu of quotes when showing dialogue, like in a play.
Correct: John: I don’t understand what a colon is.
Correct: Jane: Why not?
The colon has many uses, from showing the time to adding a subtitle to a book, however, in writing, the main function is as a gateway to something else. No matter what type of information follows the colon, there are just a couple basic guidelines: the opening statement, which is always a complete sentence, sets things up. The colon signals that there’s more to come, and the information following the colon fulfills the expectations. If you can remember that, you’ll never put your colons in the wrong place!
Using colons correctly at the University of Houston-Victoria (PDF)