The following is adapted from a reply I posted on the Writing Stack Exchange and since it’s something I’ve been meaning to write about, I figured this was a good opportunity to begin my typography Tuesday series.
There are, in fact, four different dash-like characters in typographic usage:
- The hyphen (-) is used to join words for things like some double-barreled surnames (Dan Beachy-Quick), compound adjectives (double-barreled) some chemical compounds (alpha-Amanitin) and to indicate a word broken across lines (hyphenation).
- The em dash (—) is used to indicate offset text (here—and I really mean this—is an example) and sometimes also omitted parts of words (f—k) or to indicate something has trailed off (“Are you really going to—?” she asked.)
- The en dash (–) is used for ranges (A–Z, 1–99), compound adjectives where one or both parts of the compound are multiple words (Franz Kafka–style weirdness). In some publishers’ styles (e.g., Penguin paperbacks originally published in England), an en dash with spaces on either side is used where the em dash would be used for offsetting text. It is vital, if you do this, to use a non-breaking space before the en dash as a punctuation dash should never begin a line.¹
- The minus sign (−) is used in mathematical contexts. Compare the distinction between -9° and −9°. Linebreaking is handled differently with − where the correct line break is before − but after -.²
I suppose there’s also technically a fifth, the three-em dash ⸻ which doesn’t get much contemporary usage, but was a popular device in the 18th and 19th centuries to indicate a name that was omitted (D⸻ took the train to L⸻). I used this in my story “Bartholomew L. Bartholomew” in a bit of a pastiche of 19th century style.
As for why there’s only – on the keyboard, it comes down to history. Typewriters generally greatly limited their character selection and often omitted 1 (since a typist could use l) and 0 (since a typist could use O) and had limited punctuation marks. Rather than provide distinct opening and closing quotation marks (“…” and ‘…’), we were cursed with the abominations that are '
and "
. Likewise, -
was called to serve as a replacement for the hyphen, the minus sign and the en dash while --
was to stand for the em dash. This wasn’t a big deal in that typewriters were used either for ephemeral communications like letters or were meant to be used to produce a readable manuscript for a human typesetter to work from, but in the computer age, the typescript is often used as the basis for the actual final published material and it falls on the author to do it correctly.
- The Apple linebreaking algorithm has done this incorrectly since the first days of OS X and I suspect it will never be fixed. I’ve even seen this typographical abomination appear in printed books from major publishers. This makes me sad.
- Although proper typesetting also requires some care around spacing as well.
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