Use a colon [ : ] before a list or an explanation that is preceded by a clause that can stand by itself. Think of the colon as a gate, inviting one to go on:
You nearly always have a sense of what is going to follow or be on the other side of the colon. (Compare the function of a semicolon in this regard.) You will find differing advice on the use of a colon to introduce a vertical or display list. See Using Numbers and Creating Lists.
We will often use a colon to separate an independent clause from a quotation (often of a rather formal nature) that the clause introduces:
The acting director often used her favorite quotation from Shakespeare's Tempest: "We are such stuff as dreams are made on; and our little life is rounded with a sleep."
With today's sophisticated word-processing programs (which know how much space to put after punctuation marks), we insert only one space (hit the space-bar only once) after a colon.
It might be useful to say, also, when we don't use a colon. Remember that the clause that precedes the mark (where you're considering a colon) ought to be able to stand on its own as an independent clause. Its purpose might be strictly to introduce the clause that follows, so it might feel rather incomplete by itself, but grammatically it will have both a subject and a predicate. In other words, we would not use a colon in situations like the following:
One of the most frequently asked questions about colons is whether we should begin an independent clause that comes after a colon with a capital letter. If the independent clause coming after the colon is a formal quote, begin that quoted language with a capital letter.
Whitehead had this to say about writing style: "Style is the ultimate morality of mind."
If the explanatory statement coming after a colon consists of more than one sentence, begin the independent clause immediately after the colon with a capital letter:
There were two reasons for a drop in attendance at NBA games this season: First, there was no superstar to take the place of Michael Jordan. Second, fans were disillusioned about the misbehavior of several prominent players.
If the introductory phrase preceding the colon is very brief and the clause following the colon represents the real business of the sentence, begin the clause after the colon with a capital letter:
Remember: Many of the prominent families of this New England state were slaveholders prior to 1850.
If the function of the introductory clause is simply to introduce, and the function of the second clause (following the colon) is to express a rule, begin that second clause with a capital:
Let us not forget this point: Appositive phrases have an entirely different function than participial phrases and must not be regarded as dangling modifiers.
There is some disagreement among writing reference manuals about when you should capitalize an independent clause following a colon. Most of the manuals advise that when you have more than one sentence in your explanation or when your sentence(s) is a formal quotation, a capital is a good idea. The NYPL Writer's Guide urges consistency within a document; the Chicago Manual of Style says you may begin an independent clause with a lowercase letter unless it's one of those two things (a quotation or more than one sentence). The APA Publication Manual is the most extreme: it advises us to always capitalize an independent clause following a colon. The advice given above is consistent with the Gregg Reference Manual.
We also use a colon after a salutation in a business letter . . .
Dear Senator Dodd:
It has come to our attention that . . . . .
. . . and when we designate the speaker within a play or in court testimony:
BIFF: He had the wrong dreams. All, all, wrong.
HAPPY (almost ready to fight Biff): Don't say that!
BIFF: He never knew who he was.
Click on the movie icon to the left to watch a poor-man's animated exercise on uses of the colon; click on the movie icon to the right to watch a poor-man's animated exercise on uses of the semicolon.
If you have Microsoft Powerpoint (PC or Mac version) installed on your computer, you can download the Powerpoint presentation on the semicolon (to the right) and the colon (to the left). These files take a moment to download. Click on "View Show" in the Slide Show menu and then click on the screen (or use the space-bar) to move from point to point.
Click HERE for help with Powerpoint.