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.

The Nobel Laureate of this form of punctuation in poetry was Emily Dickinson. Not only was she inordinately fond of the dash, she wrought impressive variations on it. As one commentator has noted, "Dashes [in her work] are either long or short; sometimes vertical, as if to indicate musical phrasing, and often elongated periods, as if to indicate a slightly different kind of pause. Dickinson uses dashes musically, but also to create a sense of the indefinite, a different kind of pause, an interruption of thought, to set off a list, as a semi-colon, as parentheses, or to link two thoughts together..."

In Dickinson's original manuscript of her poem that begins "Before I got my eye put out," she punctuated the third stanza this way:

The meadows—mine—
The mountains—mine—
All forests—stintless stars—
As much of noon, as I could take—
Between my finite eyes—

Until very recently, Dickinson's editors tended to convert her dashes into more standard punctuation marks, with distressingly homogenized results. Thus the 1924 edition of her work renders the above stanza this way:

The meadows mine, the mountains mine,—
All forests, stintless stars,
As much of noon as I could take
Between my finite eyes.

At the end of the first line, one can glimpse a comma-dash combo—a punctuational move that was a favorite of the Victorian age and went out of fashion not long after 1924.

Dickinson went a little jiggy with it, admittedly, but in poetry and prose alike, the dash is a freewheelin' punctuation mark. The Parenthetical Dash can stand in for a pair of commas or parentheses. The Pause Dash can take the place of a period, comma, semicolon—or nothing at all!

So when should you use the dash? Writers who deploy this mark comfortably and adeptly (rather than haphazardly) are conscious of the rhythm and dynamics of a sentence. A well-placed dash adds energy and voice. The period is sometimes referred to as a "full stop," and I think of the dash as fully a three-quarters stop. It proposes a long pause—slightly longer than a parenthesis, significantly longer than a comma—that in a subtle way calls attention to itself; as the linguist Geoffrey Nunberg has remarked, dashes are primarily found in "genres that permit reference to be made to the act of composition, whether the break indicated by the dash is genuine or artful invention " (In other words, be wary of using them in an international treaty or a scientific paper.)

To get a sense of some of the things a dash can do, take a look at these pairs of quotes.

F. Scott Fitzgerald's "The Great Gatsby":

Thirty: the promise of a decade of loneliness, a thinning list of single men to know, a thinning briefcase of enthusiasm, thinning hair.

Thirty—the promise of a decade of loneliness, a thinning list of single men to know, a thinning briefcase of enthusiasm, thinning hair.

Henry James, referring to Henry David Thoreau:

He was worse than a provincial, he was parochial.

He was worse than a provincial—he was parochial.

Mark Twain in "Autobiography":

life does not consist mainly (or even largely) of facts and happenings. It consists mainly of the storm of thoughts that is forever blowing through one's head.

life does not consist mainly—or even largely—of facts and happenings. It consists mainly of the storm of thoughts that is forever blowing through one's head.

Twain's "Pudd'nhead Wilson's New Calendar":

Each person is born to one possession which outvalues all his others: his last breath.

Each person is born to one possession which outvalues all his others—his last breath.

In all cases, both versions make sense and are grammatically correct. But the ones with the dash (the ones the authors actually wrote) seem to live and breathe, while the others just lie there on the page. Like hitting the right combination of buttons in a computer game, typing two hyphens on the keyboard - and thereby making a dash - can give your prose a burst of energy, as if by magic.

Ben Yagoda is a professor of English at the University of Delaware and the author of, among other books, "About Town: The New Yorker and the World It Made" and "The Sound on the Page: Style and Voice in Writing." He blogs for the Chronicle of Higher Education and his own blog, Not One-Off Britishisms. His forthcoming book is "How to Not Write Bad."