Let’s consider the most versatile piece of punctuation — the dash. That’s right — I’m talking about the horizontal line formed by typing two hyphens in a row. It’s the mark that — unlike commas, periods, semicolons and all the others — doesn’t seem to be subject to any rules.
You can get a sense of the dash’s versatility from the above paragraph, every sentence of which employs at least one of them. As for rules, well, there are some guidelines, but not too many.
First, make the thing the right way. There are a few ways to do it, but generally, on a keyboard, you can do as follows: previous word/no space/two hyphens/no space/following word. Word-processing programs turn the two hyphens into an unbroken line that’s roughly the width of a capital “M” — hence the official name of this punctuation mark, the em-dash. (Some publications, including this newspaper, add spaces around dashes.)
Do not call a hyphen (-) a dash — as, for some reason, computer-support personnel feel compelled to do when they recite into the telephone the characters you are supposed to enter.
Dashes are used for two main purposes. The first is what I call the Pause Dash. It more or less says to the reader, “Right here, I want you to take a breath. What you will read next relates to what you have just read in an interesting way, and I would like to emphasize it.” When using dashes this way, you are allowed only one per sentence.
The second main category is the Parenthetical Dash, in which dashes are deployed in pairs and set off nonessential elements of the sentence. When using dashes this way, limit yourself to one pair per sentence. (More than that produces confusion about exactly what is meant to be set off by the dashes, as in this sentence from a well-known piece of social criticism: “While an ethic of justice proceeds from the premise of equality—that everyone should be treated the same—an ethic of care rests on the premise of nonviolence—that no one should be hurt.”) In addition, make sure dashes are placed in such a way that, if the material within them is removed, the sentence still makes sense.
A third purpose of dashes is to indicate disjointedness. This function shows up in dialogue (“I saw Bill yesterday — wait, is that a helicopter up there? — never mind”), in prose with a stream-of-consciousness quality, and in poetry, and is subject to no rules at all.