What are the origins of terms "em" and "en" as typographic units?

Question

As you know one of the most common versions of the dash in typography are the

• en dash "–" (Unicode character U+2013 and HTML symbol –) as well as the
• em dash "—" (Unicode character U+2014 and HTML symbol —).

where the terms en and em are so called typographic units which are equal to or the half of the currently specified point size.

Furthermore according to Wikipedia

both dashes are named for the length of a typeface’s lower-case n and upper-case M respectively

as this example shows:

On the other hand practicaltypography.com states that

the em dash is typ­i­cally about as wide as a cap­i­tal H, the en dash is about half as wide. Em and en re­fer to units of ty­po­graphic mea­sure­ment, not to the let­ters M or N.

Now the question is what means em and en in typographic context really and what are the origins of these terms?

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Edit I: In German the terms "Geviertstrich" and "Halbgeviertstrich" are used instead of the terms em and en dash. A Geviert (in English quad) was a metal spacer used in letterpress typesetting. Today the is adopted for common sizes of spaces – an em quad for example is a space that is one em wide. For more details please refer to the Wikipedia article.

Edit II: Perhaps em – as a typographic unit – literally refers to the letter M and is therefore called "em" in order to avoid confusion between other unit symbols like a m (meter).

Answer 1

The em quad is a unit of spacing being the (hot metal) square body of the typesize eg. 12pt em quad. The capital M took up most of the body size. The en quad would be 6pts wide using 12pt type.

So the em part em dash and the en part of en dash both refer to how wide they are. The em and the en are units of measurement.

Answer 2

Additionally to the other answers, Wikipedia seems to answer this under the "dash" entry.

They simply are named this way because of the unit used to measure their width, as it is for the famous "one inch punch" for example!

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The en dash, n dash, n-rule, or "nut" (–) is traditionally half the width of an em dash. [...] the en dash, with its 1-en width, is in most fonts either a half-em wide or the width of an "n".

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The em dash, m dash, m-rule, or "mutton" (—) is longer than an en dash. The character is called an em dash because it is 1 em wide, a length that varies depending on the font size.

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Speculative reason:

As for why the M and N where selected as models for these units, that's probably just a matter of circumstances and naming convention.

The en and em come from the old latin while V and W (another pair of easy "doubling" letters) were not both used at the time when the first types were created (eg. W is not in the old latin alphabet). Apparently, the first texts printed were latin, that's why some types were not available (That's still a point debated today). For example, the first printers would use 2x V to create a W and that type only came in use later as the need to translate texts in other languages came and the popularity of mechanical printing grew.

So maybe the use of em and en is based on this kind of classical lingua franca trend, the first movable types and obviously are a good reference point as well. Lot of other units have latin roots as well.

This question might actually belong on the linguistic stack!

Answer 3

When I was a compositor (the good old days) setting in hot metal, I used to use a 12pt letter "M" to set the line measure in my comp stick (the tool used to set line length).

So, to set a line measure of 24 ems ("picas" is another term - still in usage) I'd use 24 x 12pt letter "M" pieces of type - or I could use 12 x 24pt pieces, or 6 x 36pt pieces - to achieve that measure. The en was used for half that unit.

With the advent of DTP, you can set your measure in whichever unit you wish - mm, cm, picas, points, ciceros!

You can still use your em and en spaces (these will be equivalent to your type size, hence a 16pt em is a 16pt witdh and an 8pt en width).

As for em and en dashes, they follow the same rule for length of the point size of type. Be aware that editorial usage varies to which they either prefer the em or en dash. Don't use typist's hyphens or double hyphens!

Answer 4

To quote Butterick's Practical Typography:

• In hy­phens and dashes … em refers to a ty­pog­ra­pher’s mea­sure­ment, not the let­ter M. The em size of a font is the same as its point size. Fonts are no longer made of metal, but the em con­cept per­sists. Dig­i­tal fonts are drawn in­side a rec­tan­gle called the em. To ren­der a font on screen, your com­puter scales the em to match the cur­rent point size. Two fonts set at the same point size will ap­pear to be dif­fer­ent sizes if one oc­cu­pies less space on its em.

• Can you de­ter­mine the point size of a font by mea­sur­ing it? No.

Imagine a block of metal type. The metal letter sticks out of the metal block. The height of that block is the point size. An em dash (or em space) is as long as that metal block is tall. An en dash (or en space) is half that.