Precision is the heart of style, so say the masters. And to be precise involves making distinctions, not only by choosing the right word for just what we want to say, but also by grouping sections of a sentence appropriately. We usually rely on a comma to keep one idea from the next, but there are two other marks of punctuation that can help in more varied situations: the colon and the dash.
The short explanation for these two devices is this: the colon specifies and the dash dramatizes. If I want to tell you something about a friend of mine, and then I want to tell you exactly what that something is, I have a choice before me how best to put those two ideas together in the same sentence. As a first design, a simple comma will get the job done without much complaint: he has to accomplish one thing this summer, to see the Grand Canyon. The comma here works from a principle of composition called apposition, the placing of one element (word, phrase, or clause) next to another to amplify the first idea. I would have certainly left you wanting to know more had I only put a period after summer and gone my way, so the comma waves you on a little further to the specific idea of seeing the Grand Canyon.
But to wave you on is to keep you moving through the sentence, and there are times when I should be careful I’m not rushing you past an interesting or surprising thought. If the very fact that my friend wants to go to the Grand Canyon is really something (and the Grand Canyon is really something), I might want to isolate that idea more sharply with a colon: he has to accomplish one thing this summer: to see the Grand Canyon. Where the comma keeps you, the reader, moving, the colon brings you to an abrupt halt—so that you and I, reader and writer both, don’t run off the cliff together. The colon has more potential energy than the comma; it’s composed of two periods, after all, so it clearly has the strength to stop you in your tracks as you come up to it. That strength ends up separating the ideas I want to communicate, and that separation produces a sharp distinction. Hence, precision.
And the dash? If the colon is abrupt, the dash is, well, dashy. Where the colon exemplifies or enumerates, the dash presents the next idea in a sentence with a little dramatic flair. The colon, which has a vertical design, puts up a hand to stop the reader in order to be very clear about things; the dash, by contrast, has a horizontal design, which serves to extend an idea rather than to bring it quickly to completion. To extend an idea means to draw out its implications, and so to write he has to accomplish one thing this summer—to see the Grand Canyon ends up presenting the second thought with a ta-da!, with a subtext, that is to say, which carries with it the suggestion and that is really something. All that dramatic strength that is part of the dash, though, is also its weakness, for a little drama goes a long way in written composition, particularly so in the more serious-minded exposition that makes up the majority of our writing. If we remember its strength, we’ll know better when to put it on display—and when not.
One last word about spacing with the colon and the dash: only one space after the colon and no space either before or after the dash. There are, be it known, no fewer than four different kinds of dashes, and the dash, remember, is not a hyphen. What we think of as a dash is more technically known as an em dash, most easily formed in Microsoft Word by typing the word before the dash, then typing two hyphens, and then immediately typing the next word—all without spaces. The computer will do the rest—except the thinking.