Today's topic is how to use colons.
One of my favorite grammar books, titled* Punctuate It Right, has a wonderful name for the colon: the author calls it the mark of expectation or addition (1). That's because the colon signals that what comes next is directly related to the previous sentence.
Use Colons After Complete Sentences
The most important thing to remember about colons is that you only use them after statements that are complete sentences. Never use a colon after a sentence fragment.
For example, it's correct to say, "Grammar Girl has two favorite hobbies: watching clouds and seeing how long she can stand on one foot." That's correct because "Grammar Girl has two favorite hobbies" is a complete sentence all by itself.
Notice how the items after the colon expand on or clarify what came before the colon. I referred to my favorite hobbies before the colon and then specifically named them after the colon. A quick and dirty way to decide whether a colon is acceptable is to test whether you can replace it with the word namely. For example, you could say, "Grammar Girl has two favorite hobbies, namely, watching clouds and seeing how long she can stand on one foot." Most of the time, if you can replace a colon with the word namely, then the colon is the right choice.
Let's go back to the complete-sentence issue: it would be wrong to say, "Grammar Girl's favorite hobbies are: skiing and reading" because "Grammar Girl's favorite hobbies are" is not a complete sentence by itself. (And, really, who would rather ski than stand on one foot?) You can often fix those kind of sentences by adding the words the following after your sentence fragment. For example, it would be fine to say, "Grammar Girl's favorite hobbies are the following: skiing and reading" because you've made the thing before the colon a grammatically complete sentence by adding the words the following.
Colons and Lists
For some reason, people seem to get especially confused about when to use colons when they are formatting vertical lists. Whenever I talk about the complete sentence rule, the next thing people ask is "But what about when you're introducing a list, like a bulleted list or numbered list?"
No need to worry. The rules are the same whether you are writing lists or sentences: use a colon when you could use the word namely and after something that could be a complete sentence on its own.
Space After a Colon
People often argue about how many spaces to put after a period (the short answer is that one space is more common these days), and the problem also comes up with colons. When people used typewriters, the style was to put two spaces after a colon; but now that almost everyone uses computers with word processing software, the common style is to put one space after a colon (2).
Colons and Capitalization
Finally, people always want to know whether they should capitalize the first word after a colon. The answer is that it's a style choice, and it depends on what is following the colon. Although the most conservative grammarians say you should capitalize the first word after a colon when the colon introduces a complete sentence, there are a lot of grammarians who say it isn't necessary. Since you never capitalize the first word after a colon if it is introducing something that isn't a complete sentence, I find it easier to adopt the less conservative rule for introducing complete sentences, and then all I have to remember is that the first word after a colon is always in lowercase (unless, of course, it is a proper name or something else that's always capitalized). But if you are writing for someone who uses a style guide, you should check to see if they have a preference because it is a style issue.
So those are the rules about colons for today: use a colon when you could use the word namely, only use a colon after a complete sentence, put one space after a colon, and don't capitalize the first word after a colon (and that's a style choice, so check with your employer or teacher and be consistent).
Colons have other uses such as in writing the time or formatting citations, but those are topics for another day.