The colon in grammar serves one major purpose: it introduces a list or an explanation (it’s also useful in time). If you see the two dots, you know what comes after is adding further information to what came before.
Let’s get formal for a moment. If you use a colon to introduce a list, you should have some word or words that let the reader know a list is coming (e.g., “the following,” “as follows”).
When I went camping, I took the following: DVD player, soap, and SPAM.
However, I’ve noticed a trend toward using colons after the verb.
When I went camping, I took: DVD player, soap, and SPAM.
Even though it seems less formal, this is grammatically incorrect because what precedes the colon must be a complete sentence in and of itself. That is, what comes after the colon is only supposed to add to a complete thought; it’s not supposed to complete it. This rule may one day change, but for now, that’s the expectation.
Colons also introduce formal quotes, but this is often relegated to more academic settings. In most of your writing, you will rarely use them this way. If you did, it would look something like this:
My uncle used to love this saying for country wisdom: “A ‘coon in the country is better than an alligator in a ship.” I still have no idea what he meant.
Give Me Detail
Finally, a colon can introduce a more thorough explanation of something.
There are two things that I know: I do not like raw fish, and I have a cramp in my foot.
There’s never been a better time to watch the meteor shower: The night sky is perfectly clear.
You could easily have put a semicolon or a period in either of those instead of a colon and not changed the meaning, but a colon keeps the two sentences more tightly bound.
Different style guides suggest different rules as to capitalization. After a colon, you should always capitalize formal quotes and explanations of more than one sentence. However, do not capitalize a list that follows a colon unless the list consists of complete sentences.
The gray area involves what to do with explanations that follow a colon and are not complete sentences. Some style guides say capitalize no matter what; some say that whatever you do, just be consistent. I won’t render judgment, but I will say that capitalizing is more often correct.
- to set off hours from minutes and minutes from seconds (3:10:45 PM)
- to connect titles to subtitles (as in this post)
- to show certain types of classification (e.g., chapter:verse, volume:page, mission: impossible)
- to set off a greeting (e.g., Dear Gertrude:)