Why Is It Called an Em Dash? An Em Is Not a Letter

In 1919, A.A. Stewart wrote a primer for printing apprentices, issued by the United Typothetae of America, and gave away the whole secret of the em dash in one line about blank metal. “An em is a square of the body,” he told them, “and varies in size with each size of type.”

A square that changes size with the type is a measurement, not a letter. The trade even had spoken names for the two units that read like a butcher’s order: Wikipedia’s Dash article records the em dash as the “em rule, or mutton dash” and the en dash as the “en rule, or nut dash.”

Why is it called an em dash?

Because the mark is one em wide, and an em is a unit of typographic measurement equal to the current type size. That same Wikipedia article states the arithmetic without decoration: “in 9-point type, an em dash is nine points wide, while in 24-point type the em dash is 24 points wide.” The name records a width that scales with the text, which is the whole reason a unit was required at all.

The em is a square of the type body

Set 10-point type and the em measures 10 points on every side. Set 30-point and it measures 30. The unit has no fixed width of its own, which is what makes it useful for describing a mark that has to work at every size.

That square was once a thing you could drop on your foot. Matthew Butterick’s Practical Typography describes the metal original: “In a traditional metal font, the em was the vertical distance from the top of a piece of type to the bottom. The en was half the size of the em.” Metal type even carried electricity: in Alexander Bain’s 1843 fax patent, current flowed only through the wires of a frame that touched the type pressed against it. Stewart’s apprentices learned the blank metal by name, since “trade custom gives the name spaces to the small type blanks and quads to the larger type blanks,” and the en quad, half the square, “is sometimes called the thick space.” Each blank sat in a drawer as a countable piece of metal, so what a printer owned limited what a printer could set, in the way supply still shapes what a small tin of caviar costs.

The capital M does not fill the square

The letter is not a coincidence. Etymonline traces the printing sense of em to “the square corresponding in dimensions to the capital M of that type,” and its entry for en dates that printing sense to 1793, defining it as “a space half as wide as an em.” The alphabet supplied the label. Clothing uses a letter as a picture instead: the T in T-shirt is the shape of the garment laid flat, body and sleeves, and stands for no word at all.

Then the label came loose from the thing. Wikipedia’s Em (typography) article files the letter-width version under a heading reading “Obsolete alternative definition,” notes that it turns up “in some older texts, but not all,” and hedges even that with “is defined, or said to have been defined.” The article’s illustration drops a capital M into a one-em square in two typefaces, and the caption states that “The M in Perpetua does not match the point size of the em.”

The measurements agree. Type designer Thomas Phinney sampled fonts for a 2011 post on point size and found that “cap size was most usually 62%-78% of the em square, averaging right around 67-70%.” A capital M set in 12-point type is not 12 points of anything. Butterick is blunter: “Em and en refer to units of typographic measurement, not to the letters M and N,” and he puts the mark itself at “typically about as wide as a capital H.”

The unit borrowed a name from the letter M and then stopped tracking it. What remains is a measurement pinned to the type size, which is why an em dash in a headline is physically longer than an em dash in the footnote below it.

Term	What it measures	Source wording
Em	The current point size	”equal to the currently specified point size”
En	Half an em	”a space half as wide as an em” (1793)
Em dash	One em	”in 9-point type, an em dash is nine points wide”
En dash	Half an em dash by tradition	”traditionally half the width of an em dash”

Repetition is what hardens a tidy explanation into common knowledge, which is also why most people have a confident answer ready for the reason the moon only ever shows us one face.

The mark reached the page before the measurement did

The dash was in English print long before anyone sized it in ems. The University of Cambridge wrote up the literary scholar Anne Toner’s research in October 2015 and reported that “the earliest ellipsis in Toner’s case studies occurs in an edition of Terence’s Andria, a play translated into English in 1588 by Maurice Kyffin.” That mark, Cambridge records, “takes the form not of dots but a series of short dashes or hyphens (sometimes three, sometimes four), also known as breaks.”

Whatever the compositor intended, what he actually set was hyphens in a row. Three and a half centuries later, typists at a keyboard with no dash key on it would arrive at the same solution.

The typewriter deleted it and Unicode gave it back

Typewriter keyboards carried no em dash key, so typists struck the hyphen twice instead. Wikipedia’s Dash article calls that two-hyphen substitute “a widespread convention in the typewriting era.” Word processors later ran the swap in reverse, since “the autocorrection facility of word-processing software often corrects these to the typographically correct form of dash.”

The machine that dropped the mark is the same one that fixed the arrangement of the keys we are all still typing on, and both of its decisions outlived the hardware. The dash got a permanent address in Unicode as U+2014 EM DASH, general category Dash Punctuation, in the General Punctuation block, present since Unicode version 1.1, carrying the annotation that it “may be used in pairs to offset parenthetical text.” In UTF-8, the encoding that keeps every ASCII character to a single byte, U+2014 takes three bytes.

The metal went away and the square stayed. Phinney describes what replaced it: “In digital type, the font’s letters are drawn on a grid, where an arbitrary number of units (often 1000 or 2048) are set to equal the ‘em’ which is then scaled to the current point size for output.” Every glyph in every font on the screen in front of you is still drawn inside the square Stewart defined, now ruled into a thousand or two thousand units a side.

The newest story about the mark outruns its evidence

The em dash now circulates online as a fingerprint of chatbot prose. Writing in The Ringer on 20 August 2025, Brian Phillips dated the theory to “around this past February” and answered it flatly: “I haven’t seen any hard evidence that chatbots, in practice, use more em dashes than anyone else.” He read the pattern the other way round, as a sign of “how reliant the AI companies are on the human writers they want to replace.”

The Emily Dickinson Museum notes that she “most often punctuated her poems with dashes, rather than the more expected array of periods, commas, and other punctuation marks,” roughly a century and a half before there was a machine to blame for it. The mark she kept reaching for is named after a blank square of metal that almost nobody working today has ever held, sized in a unit that the letter on its label has never quite managed to fill. Strictly FYI.