Why is this terrible news? Some have argued with me on Twitter that it’s more logical: - for hyphen, -- for the longer en dash, and --- for the longest em dash. You type more hyphens to get a longer dash.
My rejoinder is twofold. First, most people rarely use an en dash, although I’d like to increase that number. Second, a billion people have learned that typing -- leads to a long dash. I may be exaggerating the number, but given that Microsoft Word,* Pages, and other desktop software performs this substitution silently, it’s a widespread convention being overturned.
I’d therefore argue this is inefficient, confusing, and inconsistent. If one wants an em dash, one is now forced to type or tap three characters rather than two. It’s confusing, because typing two hyphens will no longer produce the expected result for those paying attention. It’s inconsistent, because it’s unlikely that nearly every other piece of software in use other than TeX that offers an autocorrection for -- will change its behavior to match Apple’s.
The long dash also has a particular visual identity that provides the eye with a cue to take a long mental pause, one that's not nearly long enough with the en dash.
If you're not familiar with the difference among these dashes, the simple explanation for standard American usage is:
- A hyphen, -, separates words in phrases, as in “least-used product” and is used for line breaks in books and other matter. (There’s an exception for some compounds; see below.)
- An em dash, —, is used to set off a phrase—one in the middle of the sentence—that’s not quite a parenthetical, but it isn’t so germane as to be set off by commas. It’s also used as a sort of pause to emphasize something—something important at the end of a sentence.
- An en dash, –, separates ranges for dates, numbers, and quantities—like 9–5 and January–August. It’s also used for certain kinds of connections, directions, and contrasts, such as standing in for “versus” in Lincoln–Douglas debates or in place name pairing like Alsace–Lorraine, where both are separate places being referred to together. (You can read a more elaborate explanation here.)
The inestimable Butterick’s Practical Typography, which you should memorize, has more typographic detail. On a Mac, type an en dash as Option-hyphen and an em dash as Option-Shift-hyphen. In iOS, long press the hyphen and pick the option to its right for an en dash and to the right of that lies the em dash.
This change appears in the beta release of iOS 11, so it may not wind up in the final version later this year.
A few miscellaneous em notes
What makes this more confusing is that in Australia, Canada, and the United Kingdom, typographic conventions shifted, and an en dash with a full word space on either side is frequently used instead of an em dash! Thus our English-speaking cousins have no complaint. My * above references that Word turns [any character]--[any character] into —, but if you type [space]--[space], Word follows the non-U.S. format and changes the hyphens to an en dash.
An em was traditionally the width of the height of a font, forming a square metal piece, which roughly corresponded to the letter M in that font, and is a standard unit of typographic measurement. An em dash eventually became as wide as a H rather than an M. An en was once exactly half an em. Spacing in metal type was measured as X to an em, like “3 to an em,” or as X-em spaces, meaning three of those spaces added up to the same width as an em.