I have seen the compound words "en dash" and "em dash" sometimes appear with a hyphen ("en-dash") and sometimes without. Are both the hyphenated and the unhyphenated forms correct?
There is no unanimity on the proper form of this term—at least not among U.S. authorities.
Merriam-Webster's Eleventh Collegiate Dictionary (2003) doesn't include entries for the two terms at all, but MW's Third New International Dictionary (1986) has "em dash" and "en dash." The Chicago Manual of Style, sixteenth edition (2010) shares the Third New International's preference for the no-hyphen style.
But Bryan Garner, Garner's Modern American Usage, second edition (2003) opts for "em-dash" and "en-dash" throughout his widely respected usage guide.
Perhaps the strangest preference is the one given in The American Heritage Dictionary of the English Language, fifth edition (2010), which recommends "emdash" and "endash" but also accepts "em dash" and "en dash."
Words into Type, third edition (1974) which uses the open style "em dash" and "en dash," has interesting glossary entries for em and en:
em In printing, a unit of measurement equal to the space occupied by letter M in the given font.
en In printing, a unit of measurement equal to half an em.
Why choose the letter M as the reference unit of measurement? Elsewhere, Words into Type explains:
The em. Another unit of measurement in printing is the em, which is the square of the body size of the type. The name originated from the fact that the body of a letter M in a normal face is the same number of points wide that it is high. The amount of type in a piece of composed matter is measured by ems of the size of type in which it is set; the work of compositors is measured by the number of ems they set.
So em was the original term, leading to such subsequent terms as em space, em dash, and what Words into Type styles as 2-em dash. It seems clear that M, M space, M dash, and 2-M dash would have made as much sense, logically speaking, as the spelled-out em versions of these terms; but the potential for misunderstanding by writers, editors, and typesetters of the intended meaning of a random reference to an M undoubtedly pushed the industry toward the spelling em for the unit of measurement. I have seen few instances over the years of the hyphenated forms "M-dash," "N-dash," "m-dash," or "n-dash" and even fewer of the open forms "M dash," "N dash," "m dash," and "n dash."
The use of en as a term meaning "half an em" is obviously not based on actual measurement of the width of a typical N in a typical font in comparison to the width of a typical M in that same font; nor is a lowercase n half the width of a lowercase m in any font that I am aware of. The usage may have arisen simply from the similarity in shape of an n to half of an m. Typesetters are funny that way, which may also explain why "casting off" was another standard term in pre-computer-era publishing, despite having nothing to do with unmooring a boat.
In any event, the relevant spellings of longest standing and greatest etymological sense are "em dash" and "en dash"; but if you prefer to go to battle with "em-dash" and "en-dash," Bryan Garner is a pretty good ally to share a foxhole with. And if for some reason the weirdly verblike squished-together look of "emdash" and "endash" appeal to you, The American Heritage Dictionary is at your service.