As you know one of the most common versions of the dash in typography are the
- en dash "–" (Unicode character
U+2013and HTML symbol
–) as well as the
- em dash "—" (Unicode character
U+2014and 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?
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).