The colon is one of the most versatile punctuation marks (and organs). We use it to mark time (he arrived at 3:15 pm), identify the source of biblical passages (Psalms 23:4), in mathematical ratios (the horse is running at 12:1), and, of course, in Web addresses (http://www.cjr.org), among many other places.
It’s in ordinary writing that the colon gets a bit confusing. It’s sometimes used when it’s not needed, or not used when it is. And the biggest question is how to deal with the text that follows one.
Just as colon the organ, um, moves things along, so does colon the punctuation mark. It signals to a reader that what follows is a list, an elaboration or definition, or something else that continues the thought. It says that what follows is so closely related to the first part of the sentence that a period is too abrupt, but not so close as a comma would indicate. In a driving metaphor, think of a colon as a tap on the brake before moving the car into a curve.
A colon introduces a list of items, as in: “He asked Santa for a bunch of toys: Transformers, Hot Wheels, Minecraft figures, and Legos.” That one’s easy.
A colon can introduce a long quotation or one of more than one sentence, because it says to a reader: “There’s more coming here than you thought. Stay with us.”
A colon adds emphasis, too. Take the sentence “She has decided: She won’t have surgery to hide the scar.” The phrase “she has decided” warns the reader that the next phrase is directly connected, but also warns the reader that is not a continuous phrase. If you’d used a comma or semicolon there, the reader wouldn’t have gotten the emphasis of the decision as strongly. The colon can also emphasize a single word: “Just one word of advice for you, Benji: plastics.”
Sometimes, writers will use a colon where they don’t need to. Take the sentence “His wish list included: Transformers, Hot Wheels, Minecraft figures and Legos.” The colon isn’t needed there, because the word “included” does the same thing the colon would: tells a reader that what follows is a list.
Sharp-eyed readers may have noticed that we sometimes capitalized the first word after a colon, and sometimes we did not. It makes little sense to capitalize the beginning of a list (unless it begins with a proper name), but what about when what follows is a single word or other prose?
In American English, to capitalize or not to capitalize is not a question of grammar: It’s a question of style.
The Associated Press Stylebook says: “Capitalize the first word after a colon only if it is a proper noun or the start of a complete sentence.” The New York Times Manual of Style and Usage is similar: “For consistency, capitalize what follows a colon if it is a complete sentence.”
The Chicago Manual of Style is more nuanced, and maybe more confusing. Here are two examples in its “Use of the colon” entry:
They even relied on a chronological analogy: just as the Year II had overshadowed 1789, so the October Revolution had eclipsed that of February.
Yolanda faced a conundrum: She could finish the soup, pretending not to care that what she had thought until a moment ago was a vegetable broth was in fact made from chicken. She could feign satiety and thank the host for a good meal. Or she could use this opportunity to assert her preference for a vegan diet.
Note that both of the phrases following the colon are complete sentences.
In its section on “Lowercase or capital letter after a colon,” Chicago explains:
“When a colon is used within a sentence,” as in the first example above, it says, “the first word following the colon is lowercased unless it is a proper name. When a colon introduces two or more sentences,” as in the second example above, “the first word following it is capitalized.”
So Chicago doesn’t care if what follows a colon is a complete sentence or not: It cares only whether it is a list or multiple sentences.
Chicago is clear, however, on this: “use a colon sparingly.”