Em

(n.) a single unit of printing space, equal to the current text size in points; a long dash ( — ) often used in place of parentheses or a colon

Apologies to anyone who thought they were short for emphasis, ending or enclosing, but em and en dashes, ( — ) and ( – ), are just named after the letters M and N themselves.

That’s because an em dash should rightly be the same width as an uppercase M, and an en dash the width of a letter N. (And as any good typographer will undoubtedly know, there are em spaces and en spaces that share the same size too.)

In modern typography, not every typeface follows this standard, and in fixed-width typefaces (like Courier) things become even more complicated. Nor can anyone really agree on what the rules dictating when to use em and en dashes should be. Em dashes are the more straightforward of the pair, as they tend only to be used in place of parentheses—to highlight side-points, like this one—or in place of a colon, often introducing a sentence-final element—like this one.

En dashes are more varied, and come in to play when separating numerical ranges and dates (‘January 1–December 31’, ‘pp. 35–67’), or when showing a collaboration or relationship between two different names (‘the German–Danish border’, ‘the Smith–Jones Theory’) that might be ambiguous with a hyphen. (‘The Smith-Jones Theory’ with a hyphen, for instance, would be a theory devised by a single Dr. Smith-Jones, not a Dr. Smith and a Dr. Jones working together.)

Some style guides also suggest using an en dash when adding prefixes to unhyphenated compounds (‘pre–World War era’, ‘ex–Director General’), or when similarly expanding on open compounds in some way (‘Academy Award–winning performance’). But these practices haven’t caught on everywhere, and indeed all the examples here are really only style preferences and recommendations, not hard-and-fast rules.

You could argue that this is all just too complicated, of course, and that we should just use a hyphen in all these situations. But the argument for en dashes in particular is expertly made in the classic American style guide, The Elements of Style:

When Chattanooga News and Chattanooga Free Press merged, the joint company was inaptly named Chattanooga News-Free Press (using a hyphen), which could be interpreted as meaning that their newspapers were news-free.

So if that’s where and when to use em and en dashes, what about their names?

As some of you pointed out on Twitter, ems and ens actually have a much longer typographical history than the dashes that bear their names today.

In printing, one em is equal to the current size of the typeface, measured in points (the point size of a font, as we’ve explained before, being equal to precisely 1/72nd of an inch). So the text you’re reading right now is point 18, so one em in this document would be equal to 18 points. In a different document, or a different part of this page, it would be something different; one em in the header of the main Haggard Hawks website, for instance, would be equal to 108 points.

Why have a unit that changes size depending on where it is used? Essentially, the em acts as a yardstick, allowing a printer to estimate roughly how much text space or volume they have available or have already taken up on a page in the size they’re currently working. Back in the days of movable-type printing presses, it was standard practice to fill up lines and paragraph spaces entirely—so knowing just how much of that space you had used up, and how much you had left to fill, was a useful figure to know.

The movable-type printing presses of old are also where these ems and ens earned their names. Letters in printing presses were carved into or onto individual metal blocks, known as sorts or quads. The height of these blocks both dictated the size of the line of text, and limited the maximum size of the letters themselves. An uppercase M is typically one of (if not the) largest letter in most typefaces, and so takes up more space on one of these individual blocks than any other. As a result, it gave its name to the maximum size of these blocks—and, ultimately, an area of printing space equal to a single block of text.

Half of an em is simply an en—not least because the two letters neighbour one another in the alphabet, but because a lowercase n is quite literally half a lowercase m.

The earliest reference to these measurements that we know about dates back to the late 1600s, and we’ve been using them as typographical standards ever since.